Thing is even when they had there others chances you had characters they send to kill him have there own agenda .
They send Kuma kill him but had no idea luffy was dragon son so kuma help him out .
SB they try and had ray and kuma save him .
Kuzan could have kill them but he was more concern about Robin finding a place that made her happy .
Hell in DR they had fuji who could kill him but use him get rid of the warlord system and then basically let him go .
They whole WG not try hard enough don't make sense cause every time they try something always help luffy which of course going to happen cause he the MC .
Truth.
When it comes down to it, most people have NO REASON to like the WG, and, if they can get away with disobeying them, they will.
For whatever their flaws are, Sakazuki, Kuzan, and Borsalino did not join the Marines to hunt down teenagers and little girls and kill them.
I don't think most people realize how sick of the WG most Marines are. Marines just refuse to admit that piracy can be better at upholding justice than they are, and so they keep jobbing all around the world.