It is correct that legally the trial could only have been conducted on what happened at this event. Evidence on the way the policeman had been moved sideways on other occasions after other "incidents" is not relevant.
Bu now that his past history is revealed, it seems to me quite right that we should be horrified that he was allowed back into a position where his "red mist" under stress could cause him to strike out at someone who was just blundering about and getting in the way. The man's alcoholism does not justify knocking him down, if it did, no-one would be safe.
The cotrast of his family's reaction to his death and their treatment of him when he was "thrown out" is interesting, to say the least, but the many relatives of hopeless alcoholics who have been forced to disassociate themselves from them will understand how there comes a point when it is no longer possible to help them. Having them casually killed must still be a shock for them.