Are slightly moldy beets edible after boiling?
If it were just mold, I would say you could just cut the moldy portions off prior to cooking. The smell, however, tells me they have turned bad. The mold may very well be more extensive than just on the exterior, it has probably penetrated through the stem to the center of the beet. If food ever in any way ever looks or smells "off" don't use it. Possibly getting sick or even food poisoning is not worth it. There would also probably be a very bad smell in your home while they cooked that would be hard to get rid of. Play it safe and throw them away. I grew up learning the phrase: "when in doubt throw it out." I have no doubt here, though that these beets have gone around the bend.