Actually, since Carnage only gains attack rating with levels, it's a 1-hard-point skill and can be combined with Parasite in the same build. I'm playing a solo melee necro in SP, already got 20 charms by playing with maxed Parasite for mobs and Carnage for single targets. I guess you could also make a build purely based on Carnage, probably spending the free points from Parasite in Dragonfire Oil (which I'm playing with 1 point) and relying on procs like crucify for life leech if you can get it (from rathma set for example, which btw I still can't find the damn boots to complete it), but I don't really think you would be better off in many scenarios with that setup.
The other build that was possible with massacre which was based exclusively on procs and spell dmg (with maxed shadows - now skeletal flayers) I don't really know if still works because carnage is not a faster way to trigger hits like massacre was.
Finally, about pestilence, I used it maxed all throughout the early and mid game and highly recommend it. Recently, as I got +max skills from charms and also a nice Hive Oskill from a unique amulet to replace it, I decided to respec out of it.