In stark contrast to Stephen, who at the moment of his violent death displays an exemplary meekness and compassion even for his murders, Peter is a real hothead in the early chapters of Acts. In chapter 5 he scares Ananias and Sapphira to death after they hold back some of the money gained from selling their property, and in chapter 8 he goes bonkers on Simon, the magician from Samaria who offered money for the power of the Holy Spirit:
Peter said to him, "May your silver perish with you, because you thought you could obtain God's gift with money! You have no part or share in this, for your heart is not right before God. Repent therefore of this wickedness of yours, and pray to the Lord that, if possible, the intent of your heart may be forgiven you. For I see that you are in the gall of bitterness and the chains of wickedness" (vv. 20-3).
I can't say I find Peter's response particularly reasonable. Simon probably thought that what was going on with the apostles was a kind of advanced magic, something far beyond his own ostensibly impressive capabilities (vv. 9-11). Of course, it was not magic but the work of God through the apostles. But Simon had no way of knowing this, especially since this is evidently the first time the people in Samaria are coming into contact with the apostles' movement. Moreover, we might be led to believe that Simon's previous reputation of greatness inspired a bit of jealousy in him, but if that were so, he would have tried to hex the apostles or in some way compromise their competitor magic. No, rather he believes the message (v. 13), even if it was understood from his particular context and worldview.
In light of all this, I find Simon more or less a sympathetic character, badly misguided but not obviously ill-intentioned. Peter's response is fire and brimstone of the strongest prophetic sort, where even repentance won't guarantee forgiveness by God. It seems to me too strict, too mean, too unforgiving and intolerant.
This may just have been Peter's personality. We know from the gospels that he was a fiery type: he tells Jesus he is ready to die for him (Luke 22.33), and John specifies Peter as the nameless one who the other evangelists report as cutting the ear of a servant at the scene of Jesus' arrest (John 18.10). But he also had one major lapsus: he denied Christ three times before his crucifixion (Mark 14.66-72, John 18.15-26). It seems plausible to me, then, to understand Peter's fire and brimstone as zeal inspired by his restoration to the Lord after the denial.
Through this, we may learn the following lesson: the Holy Spirit can work with people where they are. They do not simultaneously become perfectly sanctified paragons of Christian virtue; God may work miracles and convict thousands of people to repentance even through Peter, one whose temper and fire is less than what the Holy Spirit produces (cf. with Stephen's death).