Obi-Wan knew it would have been merciful to kill Ani, and he justified not killing him with “leave it to the will of the force.” It explicitly says in the novel Obi-Wan did not feel merciful after chopping off his legs though…

Imho, he didn’t mercy-kill out of spite. Obi-Wan felt betrayed, was betrayed, and wanted vengeance. So I agree with OP, it was cruel of Obi to not “finish” Anakin. Because after everything, even a Jedi can/will fall short of “rising above.” Obi-Wan wanted to betray the person who betrayed him, and didn’t rise above shit. A perfect example of how the Jedi are far from perfect.

Name a perfect Jedi and I’ll retract my argument.