Because taking Reed with him while he's dying is totally the same as skinning his girlfriend and wearing her flesh as his armor.
This Doom is complex. Waid's Doom was irredeemably evil. Waid himself has said "the truism that Victor Von Doom is, despite his villainy, a noble man is absolute crap" and that Doom "would tear the head off a newborn baby and eat it like an apple while his mother watched if it would somehow prove he were smarter than Reed."