The mentor did not willingly end himself. He was terminally ill in his last days, probably with pain to go along. That does not count. Also, he did not actually do the deed, he naivly fell for the trap of the tyrant. Even if he suspected a trap, he was compelled by his principles as a healer.
But he was a short sighted coward. If not for the trap, things would have played out quite badly for Chopper. The mentor did drink the medicine obviously to not hurt Choppers feelings, cowardly avoiding to tell him the truth. But the effect of the plant would have showed up eventually, making Chopper feel very guilty.
One could argue that he sacrificed himself to teach the tyrant a lesson. It did work on one of his minions. But sacrifice is something completely different from the cowards way out.
Even if it is about a legacy. George in It's a Wonderful Life was literally told, he was worth more dead than alive and he was facing arrest. He did not contemplate jumping from the bridge to achieve anything, he was seeking a way out.
But in both cases, the thing with the legacy is not tied to how the characters died or would have died. It is about how they lived. They could have been struck by lightning, or actually not die at all, and nothing would have changed about their legacy.
In other words, people (and their legacy) is not worth more, because said people die. Uhm. Ok. Some famous painters might differ on that - and I have even seen that one used as a plot device in crime stories.
That people do have a legacy is used as a consolation, to comfort the viewer. But in every such scenario, try to take a step back and think about how the story would unfold, if the character would not die. Are the works and the impact of that character any less, just because he lived? The moral of the Wonderful Life story, in my view, is, that he was confronted with his legacy, while still living; making him appreciate life and giving the story it's name.