That's actually a very interesting and valid theory ! thanks for sharing it !My pet theory (no pun intended) is that the serum doesn't work on someone who doesn't already have a power; their bodies aren't attuned to be able to use whatever it is that makes a power happen. Once your power is triggered your body continues to try to exploit it by adjusting your metabolism to accommodate it and has coping mechanisms to allow it to exist in harmony with the rest of the bodily functions. Think of how when boys go through puberty they often become eating machines as their body needs the calories and protein to grow and add muscle mass due to their hormones kicking in. By introducing another power it triggers the body trying to fuel whatever it is, but there is no innate buffer so the body winds up killing itself.
I've also been wondering if it is a scientific/artificial process, or if it comes from someone's power.
either power or science, it could be from someone like Mrs Powell's other daughter, which could explain why there was no real sign of struggle. even if they copied her power, there should be sign of resistance to the kidnapping. and two people with that kind of power should leave very obvious, devastating even, traces of a fight.