This is finally an argument I could give some ground to. I still disagree but it's possible they wanted him to just be a big dumb brute. It seems clear to me they wanted him to seem like a master strategist and genius though. A lot of the time he seems pretty clever and knowledgeable, it's mostly just his grand plan that doesn't make sense.
I don't think it is explained by insanity though because that is really just from the writers being inept. I mean, they baked that into his origin story, they clearly think it makes sense and weren't trying to portray it as him just being dumb for thinking that.