NPC Shadowknights won't use harmtouch when aggroed while casting a spell
 

I tested this on Guard Shiznak in Rathe Mountains. If you attack him while he's casting grim aura or a pet, he won't use harmtouch the entire fight. If you attack him otherwise, he'll use harmtouch then proceed like normal.

As an aside, in original EQ SKs would harmtouch any player (assuming they were on the hate list)in range after being aggro'd but NOT pets, unless a pet was the OnLY thing in harm touch range on the hate list, then they would HT it instead.

Seen this twice with Dragoon Zytl in WC

Also, if you attack them while they're casting a spell and interrupt their spell (stun/bash, etc) they WILL use Harm Touch when unstunned.

Also, you can root a SK mob, and when it begins to cast, walk into melee range, and it will not Harm Touch you. So less related to initial aggro and more related to casting when something enters Harm Touch range.

You mean it won't HT you while it's casting? or it still won't HT you even after the cast is finished and you're still in range?

It sounds like what you're describing is the general preference that NPCs have for PCs over any pets in their melee range. So, it's exactly what one would expect.

The second one. If you get in to melee while it's casting something it will not HT unless you interrupt it.