My proper impression of episode 20 was eaten by GAF so Here's just my prediction on the next episode.
I don't think Rider was really responsible for Maiya's death and Ilyasviel's kidnapping, at least not willingly. This is for 2 reasons. 1) They were planning to rest until sundown, he wouldn't have risked pulling that at this point unless he was forced. That point was explicitly stressed in the script trying to draw our attention to that fact. 2) He didn't know the whereabouts of the Emiya mansion. However, Kotomine had just found out its location in the previous episode and with his obsession with Kirtsugu and having acquired archer he'd be very likely to be behind the attack. Rider's appearance could in turn be due to waver being kidnapped and Kotomine blackmailing him into using one of his command spells to order the attack. This whole thing was done to draw out Kiritsugu and Saber.
[Fate/Zero] - 20
.This really wasn't supposed to happen. Everything that we've been told in the episode really led us in the wrong direction. For starters, Rider and Waver are supposed to be resting and recovering, with Rider specifically saying that he will need till nightfall to recover completely. They don't appear to readying any kind of attack. In fact, we actually fear that they're going to be attacked while they're resting by Saber, who we know is looking for them at the behest of Kiritsugu. That's why it's even more amazing when the REVERSE happens and Iri is the one caught unawareness, our expectations have been completely turned on their head.
I really like how Rider appears in this scene, or rather, how he doesn't. He attacks like some off-screen move monster and they make sure never to show him in shot fully to make his appearance and action more frightening to those who have been attacked. Also, this makes us question why Rider is operating on his own at this point because this doesn't seem like the kind of thing that Waver would plot. As has been mentioned above Rider's appearance and actions really aren't in line with what we've seen so far and so coercion makes a lot of sense.
Great stuff all round.
Having thought about it some more, I think there are some holes in this theory.
While the script does draw attention that Rider would only have recovered enough by nightfall, I wouldn't take it to mean literally that they had to wait until it is dark to attack. If Rider has a plan, he would be able to put it into action by evening. The point of the scene was that Waver was going to be resting in the middle of nowhere for most of the day, nothing more.
Furthermore, Rider had already made it clear that they should start with Saber, and while we don't know for certain if Rider knows the location of their hideout, I wouldn't rule it out either. Remember that out of all the Servants, Rider is the one who has been known to fly around the city on his own initiative, and being a conqueror he is very familiar with the need to determine where the enemy is in advance, and strike when you have the chance. The dialogue back in ep9 made it obvious that Rider is in favor of attacking head on, which isn't very different here.
But even with all these points, it is still possible that the attack was forced by a third party. What I feel doesn't make sense though, is how that is likely. Waver might not be an experienced mage, but he's not incompetent. Like all mages, he can sense the presence of other Servants, so it is unlikely that Gilgamesh would be able to get anywhere near the area without Rider materializing in defense. Kirei approaching on his own is possible, but Rider would also be able to materialize and stop him. It would be suicide for Kirei to make such a rash move against such a strong Servant.
One thing we should note is that the end of the scene with Waver concludes with him saying "do whatever you want." This is the act of consent indicates to me that he is giving Rider permission to formulate his own plan of action, and what follows is likely to be 100% Rider, and not something Waver cooked up. For all we know, Waver could still be sleeping.
Furthermore, Rider had already made it clear that they should start with Saber, and while we don't know for certain if Rider knows the location of their hideout, I wouldn't rule it out either. Remember that out of all the Servants, Rider is the one who has been known to fly around the city on his own initiative, and being a conqueror he is very familiar with the need to determine where the enemy is in advance, and strike when you have the chance. The dialogue back in ep9 made it obvious that Rider is in favor of attacking head on, which isn't very different here.
But even with all these points, it is still possible that the attack was forced by a third party. What I feel doesn't make sense though, is how that is likely. Waver might not be an experienced mage, but he's not incompetent. Like all mages, he can sense the presence of other Servants, so it is unlikely that Gilgamesh would be able to get anywhere near the area without Rider materializing in defense. Kirei approaching on his own is possible, but Rider would also be able to materialize and stop him. It would be suicide for Kirei to make such a rash move against such a strong Servant.
One thing we should note is that the end of the scene with Waver concludes with him saying "do whatever you want." This is the act of consent indicates to me that he is giving Rider permission to formulate his own plan of action, and what follows is likely to be 100% Rider, and not something Waver cooked up. For all we know, Waver could still be sleeping.
Why though? Gilgamesh taking Iri would be just as effective in regards to their overall goals. My thought on the matter was that Rider is doing this as to not involve Waver in the matter. One of Rider's concerns, it seems, is getting Waver out of this alive.
This seems like a more likely possibility. Rider is playing dirty and doing things which he does not want to burden Waver with, but needs to be done. This is also a personal agenda for him because he wants to prove to Saber that she is wrong, and it is unlikely he has much respect or concern with the people she is affiliated with.
I also find it unlikely that Gilgamesh would go along with some contrived scheme to make Rider and Saber fight each other, when he already knows they will end up fighting anyway. It would also be unusual for there to be a sudden focus on drawing Saber out, when the immediate interest of Kirei seems to be Kariya. Why would he drop his new toy and go play with something else he has shown no interest in previously? He doesn't care about Saber, only Kiritsugu.
I also find it unlikely that Gilgamesh would go along with some contrived scheme to make Rider and Saber fight each other, when he already knows they will end up fighting anyway. It would also be unusual for there to be a sudden focus on drawing Saber out, when the immediate interest of Kirei seems to be Kariya. Why would he drop his new toy and go play with something else he has shown no interest in previously? He doesn't care about Saber, only Kiritsugu.