Really? That lvl 36 Trap the Soul has an HP cap of 3380...
Indeed, the developers don't seem to understand that such instant kill effects are only fun if they have the potential to kill a monster with the first strike – and, with luck, noticeably faster than with normal damage.
For this reason I would suggest changing how these instant death effects work in general. I think a good idea would be a small percentage chance instead of a natural 20 roll, and no HP cap. The balance should come only from the chance that it procs. Another way could be a higher proc chance, but with a DC check against the player character's DC in a spell school that fits the effect – Necromancy for Vorpal and Illusion for Nightmare, for example. A third option could be that the effect has a relatively high chance to proc, but is balanced by a cooldown – and the better the character is in the relevant spell school, the shorter the cooldown, so you could expect the effect to happen more often.
These are just three suggestions on how instant kill procs could be improved, but the main goal is that they feel fun and powerful to the player without being overpowered or a must-have. If the devs have better ideas to make that happen, I'd be thrilled!
And needless to say, this isn't about making DDO easier – it's about adding fun moments to DDO. If the developers gave us such effects and nerfed the overall DPS so that we killed monsters even more slowly after such a rebalance, it would still improve DDO IMHO.