‘And set up false witnesses, who said, “This man does not cease from speaking words against this holy place, and the law, for we have heard him say, that this Jesus of Nazareth will destroy this place, and will change the customs which Moses delivered to us.”
‘Set up false witnesses' may simply indicate that they set up as witnesses the ones who had been spreading false rumours and were demanding that something be done. It does not necessarily mean that the council were involved in actually themselves fabricating evidence. And even then we must recognise that there was probably some partial truth in what the false witnesses had to say, as Stephen's own words make clear. Half truths are usually more effective than total lies which can easily be disproved. The accusations were close enough to what Stephen had said to be uncomfortable.
These false witnesses claimed that he had spoken against ‘this holy place' (the Temple) and against ‘the Law'. This would be seen as an attack on both the things that were important to the chief priests (the Temple) and to the Pharisees (the Law). They then amplified this by pointing out that what he had actually said was that Jesus of Nazareth would destroy the Temple and would change the customs which Moses had delivered to them.
The probability is that they were exaggerating what he had said rather than totally making it all up. We can compare, with regard to their statement about the Temple, how false witnesses at Jesus trial had claimed, “We heard him say, I will destroy this temple which is made with hands, and within three days I will build another made without hands” (Mark 14:58). That too we know was probably a distortion of a genuine saying of Jesus (e.g. John 2:19).
Stephen may well have let slip that Jesus had said that the Temple was shortly to be destroyed (Matthew 24; Mark 13; Luke 21), which would appear blasphemous enough to those who believed in the inviolability of the Temple. And he may certainly have given the impression that Jesus had amplified some of what the Law taught (as indeed we see in the Sermon on the Mount - ‘But I say to you' - Matthew 5) and that He had put on the Law a different emphasis from the Pharisees (e.g. Mark 7:5-23). So they might well have seen this as ‘changing the customs of Moses'. The distortions were based on half truths, which are always the most dangerous kind of lie.
He was therefore brought to stand before the council in order to defend himself. And when we consider this we must not assume immediately that the council was at fault, or even antagonistic. We must remember that the council did have the responsibility to look into charges of blasphemy. It was not the fact of the investigation that demonstrated their unreasonableness, but its aftermath.