Void magic killing otherwise immortal zerg leaders is a lazy plot device

The Overmind and cerebrates are explained as being effectively immortal. Unless killed using void powers they will instantly "reincarnate" their body. There are a few problems with this plot device.

Firstly, it's a lazy plot device and was obviously shoehorned in later in development. Tassadar wasn't initially intended to divebomb Overmind, this was suggested by the design team to add an epic climax to the script. I can't be sure, but I suspect the vulnerability to void power was added as part of this because the presentation is clunky (see my second point). The SC1 manual never once implies that the Overmind or cerebrates were immortal or could only be killed by void powers.

Secondly, the introduction of this plot point is very clunky and involves characters reading ahead in the script rather than coming to reasonable conclusions. In order:

1. Overmind tells us that cerebrates are immortal
2. Zeratul permakills Zasz
3. Overmind says only dark templar can kill cerebrates (this is a leap of logic because none of the khalai have tried killing cerebrates yet, so there's no point of comparison)
4. Tassadar (and by extension Zeratul because they're in contact) doesn't know only dark templar can kill cerebrates
5. Fenix tries and fails to kill a cerebrate
6. Tassadar reasons that only dark templar can kill cerebrates (as opposed to the zerg developing a countermeasure or something)
7. only dark templar kill cerebrates

This is an extremely clunky way to introduce this plot point. A far more elegant way to present this would be (regardless of whether it's a zerg or protoss perspective):

1. Khalai try to kill cerebrate and fail. Try to find solutions.
2. Dark templar try to kill cerebrate and succeed. Conclude that where khala fails, void power works.

Thirdly, Tassadar's sacrifice is completely unnecessary. Any of the dark templar on the surface could have stabbed Overmind to death, or Tassadar could have channeled void power through his carrier's laser canon. While divebombing Overmind certainly looks cool, it doesn't make sense in-universe and it prematurely severs Tassadar's biography when he could have lived on to be the twilight messiah and teach light and dark protoss to work together. (I don't remember where I heard this, so take it with a grain of salt, but I do recall that in an interview that the Blizz writers outright forgot why they killed Tassadar and regretted doing it in the first place.)

Fourthly, it's a hamfisted and unsatisfactory way to unite the light and dark protoss. The zerg are overpowered to the point that this plot device is the only way to defeat them, so the protoss have no choice but to work together. Their underlying philosophical differences are never resolved, and in fact never genuinely comes back to bite them. (Aldaris rebelled, yes, but he did so because Matriarch was compromised rather than because his racism came back. Ulrezaj rebels, but the other dark templar consider him and his ilk terrorists.)

Fifthly, this weakness is only ever a problem for the protoss. Whenever the terrans fight against the zerg, they never have to worry about reincarnating cerebrates preventing their victory. This is another reason why I suspect it was shoehorned in when the design team demanded the epic climax.

Now, you can certainly try to contrive various rationalizations for these problems, but that doesn't change the fact that the writing is weak.