I think there is good sportsmanship - Verbally tell opponent they do not need to shoot in final shot, but do not touch balls. This gives opponent option of shooting in the final shot should he prefer. Many times this is done in the interest of moving things along time wise. For example a scratch on the 9 in 9-ball. It is needless to grab the cue ball, set up a straight in shot and shoot it. In these cases, I don't shoot the final shot and I feel "good" about not pocketing the final ball.
Bad sportsmanship - Opponent runs table and is about to shoot in last ball. You walk up, grab the ball, and throw it in the pocket. This is along the lines of "If you're going to hurt me, I'll do it to myself first and deny you the satisfaction!". When playing these people, I would have preferred to shoot in the last ball.
What this guy did was different from the above. I was about to shoot in the 8-ball/lining up my shot, he walks up, grabs the 8-ball, moves it to the rail, and says I do not need to shoot it. (Not giving me the option of shooting it if I prefer.) My first thought was "How rude!". My next thought was; what is this guy up to? (He is not only an excellent player, he is also extremely excellent with his sharking tactics. Most of which are psychological in nature.)
So I'm wondering if...
A. He was doing this to protect himself. i.e. By his not seeing me pocket the 8-ball, in his mind, he did not really lose the game. An attempt to keep his confidence at its highest level - a mind trick on his part. (This guy hates to lose - big time!)
B. He was using a psychological sharking tactic on me (dog salivating when bell rings type of thing), that being each time I play him and am about to shoot the final shot, I should expect a disruption of some sort. This would have the effect of causing me to not fully concentrate on my final shot when playing him - keeping one eye on him to see "what he is going to pull this time".
C. He was keeping me from "warming up" or gaining any more confidence with my game. i.e. The more balls I shoot, the better I get, the more balls he shoots, the better he gets - try to arrange things so your opponent pockets as few balls as necessary.