Well i guess you'd assume that with his arms and legs off while hes on fire on the lip of a lake of flaming lava he'd be pretty rooted
Of course he never seemed to sense that he wasn't dead using that force thingie, and consdiering how close there supposed to be you'd think there'd be some connection via the force, but really, you can't kill vader, that would just be confusing
Also to finish off his unarmed and wounded oppenent woulden't be very jedi, even if it was a mercy killing i'd wildly imagine
"In the end there was only one choice. It was a choice he had made many years before, when he had passed his trials of Jedi Knighthood, and sworn himself to the Jedi forever. In the end, he was still Obi-Wan Kenobi, and he was still a Jedi, and he would not murder a helpless man. he would leave it to the will of the Force.
__________________ Some people are alive only because it's illegal to kill them
But don't you feel sorry for Anakin? When yo think about it, he was betrayed by everyone he knew, even Palpatine (even though he was too gullible to know it). The Jedi council used him as a spy, they didn't trust him, they were very abrupt and ignorant of him. He even wrongfully thought Padme was against him. As far as he was concerned, only Sidious/Palps appeared to be on his side (which Sidious/palps orchestrated masterfully). His expression when he looks up at Obi-wan, after his legs/arms are sabred, is heart wrenching - you feel so helpless for him, and feel for him ...
Ok, 3 BIG reasons!
1. Obi-Wan loved Anikan as a brother. So he couldn't kill him, just as he told Yoda.
2. A true jedi can not kill a defenseless enemy. Anikan had a burnt mechanical right arm, thats it....oh and the force too but he was to exhausted.
3. The most important point, if Obi-Wan killed Anikan then, how do you explain Darth Vador in Episodes 4, 5, and 6???
It would've been impossible for Obi-Wan to kill Anikan because of 4, 5, and 6.