First, I dislike the locally popular term "deathism", as it has built-in negative connotations, like "slut" and so thwarts your rationality before you even start thinking.

Negative connotations of "deathism" seem to me to flow entirely from the negative connotations of "death" (or, almost-entirely. Adding "ism" onto anything makes it sound a little evil, but most philosophies are named with an "ism" and that doesn't thwart our rationality). Mostly, the negative connotations flow from our actual utility functions. The idea of replacing it with a word designed not to arouse these emotions brings to mind this paragraph from Orwell's Politics and the English Language:

In our time, political speech and writing are largely the defense of the indefensible. Things like the continuance of British rule in India, the Russian purges and deportations, the dropping of the atom bombs on Japan, can indeed be defended, but only by arguments which are too brutal for most people to face, and which do not square with the professed aims of the political parties. Thus political language has to consist largely of euphemism, question-begging and sheer cloudy vagueness. Defenseless villages are bombarded from the air, the inhabitants driven out into the countryside, the cattle machine-gunned, the huts set on fire with incendiary bullets: this is called pacification. Millions of peasants are robbed of their farms and sent trudging along the roads with no more than they can carry: this is called transfer of population or rectification of frontiers. People are imprisoned for years without trial, or shot in the back of the neck or sent to die of scurvy in Arctic lumber camps: this is called elimination of unreliable elements. Such phraseology is needed if one wants to name things without calling up mental pictures of them.

Negative connotations of "Slut" on the other hand seem to flow from the utility functions of people who aren't me, and from the expectation that their utility functions are common in society, and from the self-reinforcing classification of the word as an insult as a result.