What’s interesting to me about Foxy is how unwilling he is to kill Luffy when he has the chance. He’s willing to fight with every dirty trick in the book, but despite being surrounded by all sorts of weapons he never tries to, say, stab Luffy when he’s stuck in the slow-slow beam even after Luffy’s declaration that he’d rather die than lose.
Of course, the main reason is for Oda to extend the fight and have Luffy win, but I can also see Foxy’s reluctance falling back on the main purpose of the pirate Olympics. While some accidental deaths are an unfortunate side effect of such brutal fights, the main point is to gain crew members. You can’t poach someone who’s dead.
Also, it’s really hard to gain the regard and loyalty of your newly poached crew if they saw you mercilessly slaughter the people they used to eat, sleep, and fight alongside. Part of Foxy’s weird charisma depends on the undying loyalty of his crew. So even if he’s a bad guy, he’s not a bad guy.