No, they're not killing themselves. They're looking after their interests. If you accept that D's and R's are, for the most part, one party with about the same meaningless differences as Coke and Pepsi or Ford and Chevy pick ups, then, you can see how Trump is a threat not just to the D's but also the R's. Everyone who is part of the establishments, the press, political functionaries, all the worker bee's, Trump represents a threat to them wherever they are on the ladder or at the trough. To MOST GOP'ers, professional ones, Hillary is, by far, the reasonable, rational choice. If Bernie were running against, say, Jeb, and Jeb was in danger, the D's would do their part to see that an establishment member, Jeb, wins.
Stop thinking of it as two opposing parties. They are only in who gets the top prizes. Otherwise, immigration, war, wall street, health care, all the major issues, they're the same party.