In all honesty, it's the height of idiocy to determine who you vote for based on people that you dislike and who endorse. Especially in modern US politics where both parties are big tents that will attract all manner of weird and controversial people. Equally bizarre that we would blame a candidate who gets endorsed by someone that we don't like. Then again, this guilt by association only applies to Republicans who are endorsed by people on the far right, even if the Republican candidate repudiates the endorsement, and never applies to Democratic candidates who are endorsed by people on the far left even when the Democrats welcome or praise the endorsement.
Really, nobody should care about this case. I don't know why Richard Spencer is endorsing Biden, I am skeptical of his motives for doing so. Probably just trolling or attention whoring. It's an oddity though, of no political significance.
A better person to discuss would be Donna Hylton, not just someone who endorsed Biden but an active member of the Democratic Party who is embraced by the party leaders even though she kidnapped and tortured a man to death. She is a million times worse than Spencer, who is nonviolent, and rather than Spencer who merely endorsed Trump in 2016, Hylton has been endorsed by the Democratic Party. There are numerous other violent leftists, including terrorists and instigators of riots, who are welcomed with open arms within the Democrat Party. If we are to find people on the same level as Spencer, there are countless left wing extremists within the Democrat Party who support communism, are viciously racist, and who openly advocate polices that would involve widespread violence and persecution. These people aren't on the fringe of modern leftism, as Spencer is on the right, but are an essential part of the party. Why make a big deal about Richard Spencer when there are conservatively thousands of left wing equivalents to him, and far worse, who are welcome or even dominant within the Democratic Party.