Democratic candidate who would hand Trump the election?

[Yellowhammer]

Bernie Sanders and an independent Kasich, who would lead to a reelected Trump

No Democrat would vote Kasich over Bernie.

Yes, yes they would. Many moderate democratic voters would be more in league with Kasich than Sanders. Believe it or not, some deomocrats aren't socialists and would have real trouble voting for a socialist.

[Yellowhammer]

Bernie Sanders and an independent Kasich, who would lead to a reelected Trump

No Democrat would vote Kasich over Bernie.

Yes, yes they would. Many moderate democratic voters would be more in league with Kasich than Sanders. Believe it or not, some deomocrats aren't socialists and would have real trouble voting for a socialist.

But they'd take Bernie Sanders over risking 4 more years of Donald Trump.

Just like how moderate Mormon republicans wouldn't risk 4 years of Hillary Clinton and so they all voted for Trump? No.
Some democrats would prefer a Republican like Trump to an open Marxist, just as some republicans would prefer a democrat to an ultraconservative like Ted Cruz. Sanders can never hold onto 100% of the democratic base, and expecting him to do so isn't realistic.