Well, he was stuck like 20 years in a desert hut, no? Gave him time to ponder. But then again it's nu-sw so i don't give a fuck (in the original Old Wounds comic Obi-Wan didn't gave a fuck about Maul, he was actually compelled to killed him due all the fuckery the Sith brought when Owen solved the issue with a blaster bolt in the head )
But about Anakin and Grievous
If we go by the novelization, OB1 wasn't feeling merciful, managed to get detached and left Anakin to the will of the Force. Besides he felt Palpatine getting near, had Padme on the floor and was in a hurry. Anakin had just betrayed the Jedi. the nu-SW Maul (like the original Maul) was immersed in the dark side from day one, there is no betrayal, Maul was always fucked like that.
Grievous was a jedi hunter, they were in the middle of a war, OB1 never had any personal interaction with the guy, didn't knew about his sad history and again, he was in the middle of a battle against a murderous jedi killer cyborg.