This is incorrect. There are no points given for the act of taking out a player, only your placement in the game.
That is really dependent on the situation. In most cases, if you are playing a 4 player game and you come 3rd, you be loosing points unless the player who is 4th is drastically higher than you.
See it more as, placement first, then the players ranks in the game will determine how much extra you gain or loose.
I take it that Risk uses the Elo method of ranking?
In which case - it would be good if an approximate idea of rank gain/loss could be shown somewhere in the game... perhaps putting this up during the 10 second countdown before play starts
Something like this for example:
1st Place = +100
2nd Place = +50
3rd Place = 0
4th Place = -75
5th Place = -150
Naturally the numbers won't be exact just yet - as the system updates so slowly and some players previous games may not be counted... but hopefully you will address this eventually too
You don't need additional points for taking someone out, you gain advantage to place higher by winning their cards and thus your likely to gain more ranking points.
I do want to know the formula/algorithm for how your rank is adjusted at the end of a game with respect to placement and player ranks.
For instance what if an expert player never readys for battle against beginners and below, what then? It's better for his rank to bail, but then everyone hoping to face-off against expert class suffers.