Flint Marko AKA the Sandman wasn't necessarily truly stupid or evil for that matter - it's just that he was a big dumb brute of a man who wasn't intelligent enough to resist being easily manipulated by others. For starters, he clearly didn't want to have to take part in the robbery that ended with him accidentally killing Peter Parker's Uncle Ben, but his precarious financial position and easily-led personality meant that Dennis Carradine (his co-burglar) was able to manipulate him in to taking part in it. Then there was the fact than a character who was very dumb himself - Eddie Brock AKA Venom - was easily able to convince him to joining forces in the final battle against Spider-Man. Also, in general, listening to Marko speak was evident enough that he lacked common sense. His deep, droning voice was clearly that of a big dope. Plus, when he escaped prison, the first place he went was to his own house - a move by someone lacking in basic common sense if ever there was one.
