The Incredible Melting Man(1977)

Yep, this movie is exactly like the title sounds.

Three astronauts are hit with a radiation blast during a mission to Saturn. Two are killed immediately, and the third, Colonel Steve West (Alex Rebar), returns to Earth, and naturally, begins to melt.

The usual killing spree, and assorted mayhem, ensue until he finally, just completely melts away. His remains are mopped up by a janitor and put in a garbage can. The announcement of a new mission to Saturn is overheard on the radio.

Rick Baker does the make-up.