How do I make spells non-hostile?

In Morrowind magic effects themselves are marked as either offensive or non-offensive (to my knowledge, this part is hard-coded). When you cast any spell that puts an offensive effect on any actor other than the player, the actor's AI will regard that as being attacked.

For info about which spell effects are considered offensive, check their individual articles on UESP.

Blast. Okay. Thanks

No problem.

The thought just occurred to me that you could use a dummy spell for the player to cast, and then use MWSE to replace that spell on the target actor with another one that has hostile effects.