He was also one of Japan's first Christians and a great moderniser. He was a bit like the Napoleon of Japan, for instance he saw the true potential of firearms, and included great loads of mainly Portugese (as I understand it) arquesbusses (the forerunner of muskets) in his armies. He also built the world's first iron-clad warships, long before anything alike came along in Europe.
Though to be fair, Japanese warships, iron-clad or not, weren't much compared to own, being ponderously slow and pitifully undergunned, but that's all behond the point. It's the idea that counted.
Personally, I think he was pretty cool. Where the whole 'evil warlord' thing came from I don't know.