I don't think that would work as a reason because of what ZE_501has already said: it wasn't the merciful thing to do. There is also a big difference between this situation and Dooku.
Yes both was at the end of a duel, but Dooku only lost his hands but was otherwise uninjured. The correct move for Anakin here was to arrest him so he can stand trial (exactly as what Anakin said Windu should do to Palpatine).
For Anakin vs Obi Wan, Anakin lost his limbs and was being burned by lava. Leaving him alive would have been torture. If Obi Wan meant to leave him alive, then he should have captured him and brought him to stand trial (like what Anakin should have done with Dooku) or killed him outright (like he did with Grievous and Maul).
So it's either Obi Wan was a sadist, or he thought Anakin was good as dead. He just left Anakin there, writhing, burning and screaming: