Doom should not be a two-dimensional cardboard baddie. In the reality, statements along the lines of "s/he would never do that" are never absoulutes. People are too nuanced and complex for that. That person who is "always" practical and frugal will be willing to exorbitant prices for one specific thing because of its sentimental value. That person who is "always" level-headed and rational has his one or two sensitive topics that he simply cannot be rational about. Reed has always driven Doom to extremes of behavior. In one of the last Doom stories Lee and Kirby wrote, he was pushed so far off the edge that he even tried to blow up an entire Latverian town full of innocents just to kill the F4.
This is perfectly believable behavior on his part.