The conversation about jellyfish at the start of the manga proper is actually about Negi, explaining how he could have died in the No Asuna!Timeline and this timeline - his mastery of Magia Erebea rendered him immortal, and unable to die by illness or age, but not invincible, or unable to die by sufficient injury. Naturally, this hints that there is, in fact, a Big Bad of some sort that did said killing, on the basis that death by injury typically has an injurer in this genre (probably related to the death of the parents, as car crashes usually don't involve being nailed to the ground with swords). All conveyed via metaphor and implication that the characters don't even know the relevance of.
Touta's Magia Erebea seems like an Ass Pull because he gains a large amount of power with virtually none of the drawbacks Negi had. But all those drawbacks came from Negi being purely human; as a vampire, Touta does not have those weaknesses. He also isn't constantly absorbing High Ancient-level magic every five minutes, and doesn't have Negi's major psychological hangups, so it shouldn't go out of control and turn him into a ravening monster as easily.