Ok I have ran some tests and actually got the values that I expected.
If you are interested I can show you the graphs
So indeed your formula rises exponentially, that is because if you work a bit with that formula:
heads + (kills+deaths)*(kills/deaths) you get this :
heads + (killsē/deaths + kills)
That square is annoying so I removed it.
As a result I get : heads + (kills/deaths + kills)
And this does give me nice results and it doesn't have the problem of going negative(which is risky).
Perhaps not fully as you had intended but now we have no problems with mathematical behavior.
now we have to make some changes to the database and I'll start implementing this.