It's difficult to explain very briefly why he is hated because he's been ruling the country for the last 13 years and throughout this period he's done a lot to piss off many people but I'll do my best to mention a few. In the 90s he served as the mayor of Istanbul from an Islamist party and it was a shock to all secular/liberal people when he won the election in 1994 against a popular left-wing musician. His popularity grew as a mayor and ironically in 1999 he served prison time for reciting an Islamist poem at a rally. In 2001, he an a few friends left the Islamist party and founded a new party (AKP) and claimed that they aren't political Islamists anymore, but instead conservative democrats in a similar fashion to the Christian democrat parties in Europe. After the 2002 elections his party won enough seats to form a single-party government and he's been running the country ever since.
In fact, in their first term AKP really looked much more like a mainstream conservative-liberal party rather than an authoritarian-Islamist one that we have today. This was probably because there were lots of liberal center-right figures in the party at that time which still could have influence and although Erdogan's bigotry was showing here and there, it wasn't a big deal because there was progress being made toward democratization and EU membership.
After the first term things began going south gradually, and especially since 2011 Erdogan controlled pretty much everything while at the same time doing his best to make the population more conservative than it already is. Here are some examples of how bad it is:
On women's rights: he argued several times that women and men are not equal, that he values women only as mothers, that they should bear at least 3 children, that they should not have abortions (although abortion is legal in Turkey, after Erdogan's stance on the issue became clear, many hospitals were discouraged from performing abortions), and should not even have c-section births.
On Syrian Civil War: he openly supported 'moderate' rebels, who turned out to be nearly as bad as ISIS. He sent weapons and aid to those Islamist rebels and when this was made a news story, the journalists who wrote it were jailed for 'espionage' (they were later released thanks to the Constitutional Court). Thanks to Erdogan's involvement in the war in Syria, there are now nearly 3 million Syrian refugees in Turkey, many of whom are on the streets or working illegally for very low pay.
On freedom of speech/press/Internet: You've probably heard it on the news sometime that Twitter was blocked in Turkey for a while. There are currently more than 50,000 websites blocked in Turkey by simple court orders and it's very difficult to reverse those. Most people try changing DNS servers or using VPN's to bypass those bans. Most of the press is also controlled by pro-Erdogan people because popular TV channels and newspapers were either purchased, 'seized' with a court order and then given to a pro-Erdogan boss, or coerced into changing their policies. And about the freedom of speech, the reason why this post exists: there are nearly 2,000 lawsuits opened for 'insulting' Erdogan and this is only one thing to worry about when speaking in public in Turkey.
On Soma mine disaster: In 2014 there was a mine disaster in Soma, Turkey in which more than 300 miners died. It was the biggest mine disaster in Turkey. The survivors and the relatives of those who died in the disaster were of course angry because the company ignored many regulations which could have saved a lot of lives. So, when Erdogan visited the town, instead of trying to console the people, he tried to punch a man shouting anti-semitic slurs. And his aide, who was approached by a relative of a miner who died in the disaster, kicked him while he was held on the ground by the military police.
On Gezi protests: By 2013, Erdogan's authority was pretty much established and he didn't see the need to consult the inhabitants of any place before ordered development plans, huge urban transformations, destruction of parks and forests. In the summer of 2013, when inhabitants of Istanbul witnessed that the trees in one of the few urban parks in the city center - the Gezi Park - were being cut down, decided to protest and set up tents in the park in order to prevent the demolition. They met with a very violent police response and their tents were demolished. Their friends came in to support them, and the police struck harder. Thus the protests grew rapidly and millions of people were on the streets in most Turkish cities. Police brutality during the protests claimed 11 lives (and gas canisters aimed directly to the face made many people blind), including a 14-year-old boy who was in a coma for almost a year before he finally died. Erdogan claimed full responsibility for the police brutality ("I ordered the police. So what?") and argued in a rally that the 14-year-old boy was a terrorist and made the crowd boo his mother.
On education: Lots of secular schools were transformed into religious "Imam-Hatip Schools", which used to be vocational schools for imams but now simply a way to impose religion on kids and to separate boys and girls from an early age.
On corruption: In late 2013, Gulenists, a powerful religious group which used to be Erdogan's ally but they had a falling out, exposed several phone recordings and other evidence that revealed a huge corruption scandal that involved Erdogan, his family, his close friends, state-owned banks among others. Erdogan responded with a purge in the police force and judiciary and recently seized the group's newspaper (the most circulated paper in Turkey).
On Kurdish question: Erdogan's government initially tried to make peace with PKK, Kurdish insurgent group and the efforts continued until 2015. But when the pro-Kurdish party won 13% in elections, Erdogan, who believed that his party should have been the only one benefiting from peace, broke all peace negotiations and started a heavy crackdown on PKK and the urban warfare is still going on in some Kurdish towns with many civilian casualties reported. I've already mentioned that in Syria, Erdogan supports the Islamists, and he tolerated ISIS because ISIS fought the Kurds in the north, which was seen by Erdogan as the bigger enemy.
These are only some of the things that came to my mind, and specifically what made me hate him. Someone else can come up with completely different reasons to hate him. Also keep in mind that along with what he does, what he says matters a lot too, and most of what he says is very divisive and hateful and I'm very concerned about the future of my country. Even if he died tomorrow, the wounds he inflicted would take decades to heal.
