Why did he do it? Deadly Toronto van Attack

I wasn't usually into news but waking up in the morning and receiving a message asking me if I have seen what happened in Toronto was a bit of a shock for me. I can't even imagine myself walking in that same street wherein many people oblivious to what's happening around them died because of just one guy! A guy who knew that his actions could kill innocent people who were just walking along the sidewalk which was supposed to be the designated area for them to be safe from the cars that could cause accidents. My question is why? Why did he do that? Was there someone threatening him? Was he on drugs? There's already natural phenomena here on Earth so why kill people? Why kill innocent people whom you do not even knew personally in the first place? This is not just in Toronto but to all around the world. Accidents are accidents but different from murder.