I support Sanders based on the policies he's proposing, the things he cares about roughly align with what I care about. The label you put on it doesn't matter too much to me.
If you're a Sanders supporter that thinks Trump supports you better than Clinton, I'm honestly curious what made you like Sanders in the first place.
On income inequality
Sanders Top tax rate 40%->54%, min wage 7.25$/hr -> 15$/hr.
Clinton 40% -> 44%, 7.25$ -> 12$
Trump 40% -> 25%, federal minimum wage abolished.
On climate change
Sanders wants harsh carbon taxes, both clinton and sanders want to continue clean energy subsidies put in place under obama, Trump calls climate change a chinese hoax and wants to get rid of the EPA
On money in politics
Sanders and Clinton have both explicitly said they would appoint justices that would overturn citizen's united, and have plans for campaign finance reform. Trump has said he doesn't like superpacs, but has put forth absolutely nothing on finance reform, and has promised to appoint a conservative justice. For context, citizens united was a 100% partisan split, with every conservative leaning justice for and every liberal justice against.
On foreign policy
Both Clinton and Sanders support continuing foreign aid while cutting our military spending, Trump has proposed the opposite. And while many people criticize Hillary for being hawkish, Trump literally came out supporting expanding torture and killing the families of terrorists. Both war crimes. Thank god he hasn't had the opportunity to be hawkish.
This is getting kind of long, and I'm trying to condense it, but I could write a few sentences on plenty of other big issues right now, including abortion, health care, gun control, college affordability, etc. On nearly every issue, Hillary is Bernie-lite.
I can understand people who don't want to settle for Bernie-lite, and are not going to vote, or people who want to vote third party, but there is essentially no way that someone who supported Bernie on a policy basis would find themselves best represented by Donald Trump.