Yes, kills are worth it even in turf war. Think about what the opponent is doing and could be doing, and then you can realize the opportunity cost for having died.
For example, let's just use you as an example:
To not think extremely far into the future, we'll divide up what you'll be doing with the present and the future.
Presently can only be one thing, because it is of course the thing you're currently doing. So for the example, let's say there's no one around you know of and you're presently spreading ink.
In the future, you realize the opponents are going to push into the area soon, so you decide you'll be doing one of a few things depending on the need of the moment.
A) Pressuring
B) Punishing (killing people who overextend)
C) covering their backs, so they don't have anywhere to go to (flanking without immediate punishing)
D) covering more turf
Now let's say an opponent charger shows up right as the future is about to become the present, and shoots you.
Now you are forced into what will be labeled as action E for a time: respawning and jumping or moving back up. Essentially, for dying you lose out on your current present and whatever future options you could be making the present that could be crucial to the team for the length of E. A, B, C, and D are all now impossible until you move back up, and may stop being viable options by the time you are able to.
So, to sum it up, when you die you stop contributing what you are doing and many future options become unviable by the time you have completed situation E. so Dying is worse during crucial points when you have more and more important futures or are performing important jobs. The higher the opportunity cost was for the person who just died, the more important the kill was.
Of course, you also must pay an opportunity cost FOR killing, as all the other options you did not pick are lost to the now current present. You essentially pay an opportunity cost to make your opponent take a bigger opportunity cost, and if you screw up and die yourself, then it's even worse, because your opponent didn't commit to the killing option as hard as you did, meaning he would likely have lost less time in which he could be doing other things than you did going for a kill, and are now also losing due to respawn + move up.
an entire team being pushed back to spawn by the entire other team is the extreme result of what can happen if your team is forced to pay several opportunity costs over and over from dying, as they collectively lose more and more compared to the other team until they're backed up in spawn.