As well all know, Tony suffered from panic attacks/depression etc. And almost all of his decisions stem from allowing his judgement to be swayed by paranoia, ego and sentimentality. He wasn't stupid in any way, quite the opposite. He was able to think 2/3 steps ahead on countless occasions. But in my opinion, even when the panic attacks subsided, there remained a mental flaw that meant he wasn't ever really cut out for the top job.