Liquid was on an inferiority complex driven warpath. Ironically even though he resented Big Boss for being stuck with the "inferior" genes, he ended up proving BB's point that a person is more than their genes, by doing all the things he did.
MGS4 is a controversial one. It definitely improved on MGS3 mechanics, but a lot of the game's segments weren't very demanding in terms of stealth (except for the Shadow Moses part, that was definitely awesome and nostalgic). It also trivialized a lot of stuff from earlier MGS games with the whole NANOMACHINES excuse being used out the ass, it was also the most ridiculously cutscene heavy. I know MGS has always been full of cinematics, but MGS4 went overboard.
And I know it's the unpopular opinion but I actually enjoyed MGS2, at least from a story perspective. The themes like memetics, information control, etc were really interesting. That scene near the end with the Patriots AI was some thought provoking shit.