Well with a Necro-Pet its actually the Necro killing you himself, as he channels the spells through the pet.
If that's fair or not doesn't matter, the Class always was like that, until it was completely changed on Live to a class, thats more like a Cabba with Shapeshift then.
And it also doesn't help to whine about the pet/keep-issue, it's a coding-thing that probably would lead to even more problems, if it was changed to prevent pets as well as any monsters from moving through obstacles. would need a completely new pathing.
And since Phoenix itself is a freeshard, i do not think you want to pay the manpower to really change that, do you?
So in all honesty, yes, it's sometimes annoying, but DAoC always was about RvR and not Player-vs-Player, there always were some classes, that in a 1on1 are all but "fair" or "equal", so just don't go alone, have somebody to kill the pet, or stay next to others, that simply can kill it down, and its the pet-caster that has to regularly spend his power to recast his pet, over and over again
Addition:
And just to make that clear, all realms have a slightly... uhm... less balanced class thanks to SI..
Necros, ofc.. but in certain situations you also dont want to face a Bonedancer nor an Animist alone, or even in a small group, it always depends on the class you are playing as well as the situation.
Just think of the Shrooms placed inside PoC, yay, fun.
Or think of Insta-Lifeleech + several-Healer-Pets when you are alone yourself..
That's just how it goes, and how it always was. DAoC is no W**-game, where only exist two Realms that are completely equal to each other.. all 3 Realms are different with different classes and combinations of abilities, ofc you never will be able to balance that on every 1on1, and its also not supposed to be balanced on 1on1.