Is the seemingly violent Jesus in the book of Revelation in contradiction to the nonviolent Jesus of the gospels? Is it true that Jesus came the first time as a lamb but he's coming the second time as a lion?
Actually, the whole book of Revelation is centered around glorifying the revelation of Jesus as the lamb. It mentions him being a lion only once and in order to subvert it to show that Jesus' lion-like triumph is through being the lamb.
One example is the "wrath of the little lamb" (the Greek "arnion" = lambkin, or a little innocent lamb). Typically we read that as "the wrath of the Lamb" as if it's some huge angry Lamb destroying the nations, like Lambzilla, but what we actually have here is an outlandish idea aimed at subverting our understanding of power. The pompous, monstrous beast is defeated by a little lamb. The nations are running in terror from the wrath of the little lamb. The little lamb has 7 horns and 7 eyes, reimagining the perfect power and wisdom of God as the little lamb. Paul says that the cross is the power and wisdom of God. This is foolishness to a world that believes power and wisdom is something like Caesar. But God overthrows the powers of the world through a little lamb.
In the apocalyptic scene in Revelation where the Logos of God is revealed, he is on a white horse coming to make war, and how does he make war? Not from a sword in his hand, but from a sword coming from his mouth. Again we have a subversion. This isn't literally a mouth sword. Jesus is striking down the nations with a message. His robe is dipped in blood before even entering into battle. It's his own blood. So the nations are under the spell of the violent domineering beast and Jesus strikes them down with the message of self-sacrificial love and shedding his own blood. This is also how his followers triumph over the beast as well, by the blood of the little lamb and by following his example of laying down their life, and by their testimony of this lamb.
The book of Revelation is subverting apocalyptic imagery in light of the revelation of God in the cross.