Yordle Hunter Tahm Kench

I think you are quite wrong with that. If you looked on the League wiki it shows Tahm as an ally to Veigar, meaning what you just said is false. Plus the Wiki explains why all the yordles other than Veigar are shown as rivals to him: "All Yordles have encountered Tahm Kench at various times. This is because of their races specific nature that makes them more perceptible to inner demons when socially outcast. When not, they are hard to be manipulated, which is why almost all Yordles are seen by this demon as rivals."