Knowing of the various shady, and sometimes illegal, ways in which keys sold on sites like G2A may have been acquired, and having no way of telling the origin of keys, makes me uncomfortable with the idea of buying games from them.
I suspect the majority are gray imports, which I'm iffy on. Steam's 60 is ridiculous, but RU/BR prices tend to be too far in the other direction. I would like to pay a price I reckon is fair (GMG usually provides). Because I like games and want to support the people producing them. And I don't want to feel like I'm encouraging a proliferation of bullshit practices. If I want to go around complaining about microtransactions and stuff, I can't really demand they lower new game prices by 40% at the same time.
G2A's marketing, aside from that disgusting mobile ad someone posted, isn't in itself a problem. I am disappointed in streamers and tubers who do promotional deals with G2A though. Like, you use the games by these publishers and developers to produce video content which makes you money. And point viewers to G2A, which makes you money, and the pubdev less/none. Kinda shitty.