Fairly sure I've already posted why in a similar thread. Oh well, I'll post what got me to start considering Islam as being false. When I was 9-ish I had confirmed that I was the only person alive. I was put in this world by some scientists as a sort of experiment. The world literally revolved around me; people wouldn't perform any actions unless I was around. Kids wouldn't play, adults wouldn't argue, teachers wouldn't shout; they would just remain inanimate, possibly being moved by the scientists from one place to another. Fast forward around 2 years, as if the scientists were trying to delude me from escaping this experiment, my grandfather passed away of cancer. It was a very odd day, I'd been feeling fairly down for no reason, the school bus had broken down on the way home for more than 3 hours, and when my mother told me over the phone I kept thinking about all the things I could have misheard. Similarly, my two other grandparents had fairly odd deaths, like my grandmother's wooden sofa that my cousins and I had used as a semi-playhouse for years on end breaking to pieces on the night of her death.
Things like that were the reason why I believed in the existence of Allah. That, combined with the fact that Allah is fact in all places in Egypt, formed this sort of impassable terrain; a wall of blind faith. For so many years after realizing that it's probable that other people are conscious too I struggled with the concept of hell. The number of non-Muslims who have existed throughout history and will continue to exist for tens and hundreds and thousands of years to come is without a doubt up in the hundreds of billions and more. Note that the Muslim heaven didn't interest me to begin with. So all the non-Muslims will burn in hell. Almost everyone responsible for today's civilization and how much progress we've made is going to burn in hell for eternity. Meanwhile, the Muslims are victims of their own fate. Whatever happens to them happens because Allah has willed it. Specially those damn 16 year-olds in the traffic accidents and murders, it's not that we have an impressively weak police force that does nothing to maintain road rule and prevent crime but instead threatens and locks up the righteous, it's that Allah wanted to save those 16 year-olds from going corrupt later in life.
Confused, I started digging into Islamic history, attempting to see how Mohammed's action look if I was to imagine that Allah didn't exist. Stupidity, thievery, murder, genocide, rape, slave trade, torture, verses that seemed a little bit too convenient, and contradictions between the hadiths and verses and other verses.
And (remember) when you said to him (Zaid bin Hârithah; the freed-slave of the Prophet ) on whom God has bestowed Grace (by guiding him to Islâm) and you (O Muhammad too) have done favour (by manumitting him) "Keep your wife to yourself, and fear God." But you did hide in yourself (i.e. what God has already made known to you that He will give her to you in marriage) that which God will make manifest, you did fear the people (i.e., Muhammad married the divorced wife of his manumitted slave) whereas God had a better right that you should fear Him. So when Zaid had accomplished his desire from her (i.e. divorced her), We gave her to you in marriage, so that (in future) there may be no difficulty to the believers in respect of (the marriage of) the wives of their adopted sons when the latter have no desire to keep them (i.e. they have divorced them). And God's Command must be fulfilled.
There was also a hadith about Aisha commenting on the convenience of the revelations which I will edit into this post i I find.