Can a anime ending before the manga that it's based off of hurt the legacy of the anime?

No, it wets the appetites of those who watched it, making them want more, either in the form of another season, or the source material, if the need is great enough. Anime have standard lengths and everyone watching them already knows this. Anime that tell the full story are very rare and so people don't expect the run-of-the-mill anime to do that.

People won't be bitter about an anime ending before it adapted the entirety of the source material, they'll be bitter if there is no sequel, if the anime was actually worth one in their eyes. That said, if the anime has an original ending, then that is a whole other matter. And it's enough to spend some time in the anime community to know all that. It's common sense basically.

Also, you did ask this same question before.

yes it can.

look at the fuster cluck that is tokyo ghoul. 2nd season was so awful that it put a poor taste in the mouths of many

1. You've already asked this question.

2. No. The concept of a sequel exists for a reason.