The value in awarding the Nobel Peace Prize to warmongers is — and the awarding committee is very conscious of it — that it has enough cachet to actually motivate warmongers to stop warmonging and go for the prize (and a relatively well-off, safe and calm retirement).
It may not be fair, but the people who don't die because of that effect won't complain.
There are also instances of relatively peaceful people who got the prize and became warmongers, e.g. a certain president of the us, who sent 500 drone strikes into Libya in a single year after winning the prize (among other military adventures)
If there are as many examples of people receiving the peace prize then going on to be warmongers as there are the reverse, it suggests that the peace prize isn't really doing anything here and we're just looking at the base rate of leaders switching between peaceful and warlike actions.
It may not be fair, but the people who don't die because of that effect won't complain.