Jesus was a preacher who spread his message of peace, love, and above all, justice, across Judea during his actually average-length life for his time. He was remarkably charismatic, attracting a small but incredibly loyal group of followers, and he was said to perform miracles, although it's worth noting that miracles weren't terribly uncommon at the time. Jesus' message was particularly resonant to the poor and downtrodden, and pretty radical in its anti-authoritarian stance. He said it was easier for a camel to get through the eye of a needle than for a rich man to get to heaven. He said the meek were blessed, that the last would be first and the first would be last. All of which was kind of threatening to the powers that be, who accordingly had him arrested, tried, and then executed in the normal manner of killing rebels at that time, crucifixion. Also, just to put this question to bed, the Romans crucified Jesus because he was a threat to their authority.