Depraved people in their natural state find that infuriating – especially when it is their proselytes who are turning away from them towards the Truth. (You made the *assumption* that God was trying but failing to save those who were persecuting Stephen, but that is not actually in there.) So in Acts7:51, Stephen was just calling back to Jesus' words in Matt23.
Matt23 Then Jesus spoke...2 saying: “The scribes and the Pharisees...4 For they bind heavy burdens, hard to bear, and lay them on men’s shoulders; but they themselves will not move them with one of their fingers. 5 But all their works they do to be seen by men...13 “But woe to you, scribes and Pharisees, hypocrites! For you shut up the kingdom of heaven against men; for you neither go in yourselves, *nor do you allow those who are entering to go in*...15
“Woe to you, scribes and Pharisees, hypocrites! For you travel land and sea to win one proselyte, and when he is won, you make him twice as much a son of hell as yourselves.
...31 “Therefore you are witnesses against yourselves that you are sons of those who murdered the prophets. 32 Fill up, then, the measure of your fathers’ guilt. 33 Serpents, brood of vipers! How can you escape the condemnation of hell? 34 Therefore, indeed, I send you prophets, wise men, and scribes: some of them you will kill and crucify, and some of them you will scourge in your synagogues and persecute from city to city, 35 that on you may come all the righteous blood shed on the earth, from the blood of righteous Abel to the blood of Zechariah, son of Berechiah, whom you murdered between the temple and the altar. 36 Assuredly, I say to you, all these things will come upon this generation.
37 “O Jerusalem, Jerusalem, the one who kills the prophets and stones those who are sent to her! How often I wanted to gather YOUR CHILDREN together, as a hen gathers her chicks under her wings, but you were not willing! 38 See! Your house is left to you desolate...