I never have figured out the motive for Robert Culp to murder "the kid"?

Yes, that's something which has always bothered me. Motive. I can find no (clear) motive. Upon watching this, I usually get a sense that Hanlon is attracted to Eric Wagner's wife, Shirley. There is also a sense of Hanlon desiring to completely control the business. By eliminating Wagner, Hanlon can have both the wife and the business to himself?

As I said, these are not clearly shown motives, only supposition on my part, in an effort to fill in that which the writers left out.

For a pretty complete perspective and analysis of this - and all the other - episode(s), check out this site: https://columbophile.com/2017/02/12/episode-review-columbo-the-most-crucial-game/

I can't say I ever found it difficult to just think that he did it to stay in control, the victim could have just not bothered with the team and let it become third rate, or he could have sold it, then the killer would probably be out of job.