There was no implication he left him expecting him to die.
Part of the larger picture is that Kenobi knows who is truly responsible for everything going on. He kills Vader, then what? It doesn't magically solve anything. He knows Palpatine has used Maul, then Dooku, then Vader. Kenobi killed Maul (well, he thought), and it didn't solve the problem. Kenobi was involved in the death of Dooku. Didn't solve the problem. He knows killing Vader might temporarily deal with the immediate situation, but it won't make a difference in the grand scheme of things. Based on past experience, Palpatine probably has someone else waiting on deck or will quickly get someone, and then Vader will just be another in the list of Sith flunkies Kenobi has seen die without really changing anything. The Inquisition is out finding new Force sensitives, and killing Vader won't stop that either.
It might have a made a difference years earlier, when Vader was setting up the Inquisition, training Inquisitors, and hunting the surviving Jedi, and Palpatine's hold on power might have been a bit more fragile so taking out his apprentice might have signficantly set his plans back, but that window has closed. Killing Vader won't stop Palpatine or the control he wields through the Imperial Forces, and won't prevent him from taking on another apprentice.
An apprentice who Kenobi might not know and thus not have any advantage over, unlike Vader who Kenobi has defeated, and who he knows.