Killing anyone requires a Compassion check if it's high enough. At 5 dots you have problems hurting people at all and most attacks in combat will trigger a roll, at 4 dots you're reluctant to kill any but your greatest enemies, at 3 dots you have problems killing people whose actions seem excuseable, at 2 dots you avoid killing innocents, and 1 dot makes you a murdering bastard.Why would killing the Necromancer require a compassion check? I thought that was only for innocents.
As a madman, his actions might very well be excuseable. If he were sane it'd be a Compassion 4 prerequisite, but he's not, so pity lowers it down a bit. Of course, having a high conviction would negate this.
Anyone with Valor 3, incidentally, finds that discretion might not be the better part of valor. Valor 3 means that you at least take a swing before you retreat, while Valor 4 means that retreat is only an option against a clearly superior foe, if that. Valor 5 means no retreat, however small, and certainly no hiding behind the spellcaster. Come to think of it, hiding behind the spellcaster is Valor 2. Valor 3 means hoping he stops casting whilst you gut the Necromancer.