Honestly, who knows how the body of a servant "dies" seeing as it really is just a "spirit" with a body and a life force supplied as mana from their master. Kayneth said they were "ghosts" of the past, so any and all feelings including grudges from the past could have contorted the way Lancer manifested himself at his last moments. That what I thought. Maybe even present feelings. Not sure, but Berserker's eyes did go all white in it's final moments in trying to protect Illya In UBW.