Well, two things there.
The bloodlust for battle IS hereditary in the Saiyan genes. That's a big part of their character, their obsession with fighting. They're savages. Goku just happened to get lucky and lose some his savagery, but none of his lust for battle.
So he's a nice guy, but he still can't give up the chance at a good fight.
Also note that, ON THE WHOLE, when he gives villains a chance and beats them into submission, they tend to eventually become allies. Often because they can't beat him or have a common foe, but still, allies. This applies to Yamcha, Tienshinhan, Piccolo, Yajirobe, Vegeta. Arguably Krillin was a rival too, but that was when they were little kids.
Freeza is the first opponent that was completely beyond repetance--note that Goku DOES the usual "I've beaten you" thing and lets him leave...but Freeza decides to go for the cheap shot and Goku's forced to nuke him.
That being said, leaving Buu to the younger generation is different; he specifically says it's not for his own lust of battle--it's so the Earth has a protector when he is gone for good. Gohan was doing an ok job, but after Fat Buu awoke, Goku thought Gohan was dead.
If he had his own interests in mind, he would've gone all out SSJ3 and wiped Fat Buu out. But he wanted the kids to learn how to fight and protect Earth, so he gave them their chance. It worked with Gohan before--Cell would have eventually beaten Goku and then the Earth if Goku hadn't stepped aside to let Gohan take over.
Goku's not the smartest person by a long shot, but he has pretty consistent and clear reasons for what he does.
