«Karl's War» by Spark Neil

Karl’s War is a story of love, revenge and redemption. It begins in the tinderbox of high tension and conflict that was Berlin when Hitler came to power. It’s the story of how one man overcomes hostility and self-doubt to fulfil his quest for happiness. And it's the story of how three young men oppose the Nazi regime.
Karl Bauer is a young man from a Nazi-sympathising family. He wants to escape from his brutal father and tormenting brother, Günter. Karl’s father forces him to join the Nazi paramilitary stormtroopers where he meets Reinhardt. About the same time, Ben Herschel, from a liberal Jewish family, befriends Karl. The Herschels are persecuted and flee to France. Karl vows to join Ben there.
When Reinhardt’s male lover is murdered during the Night of the Long Knives, he vows revenge. The trio meet in Occupied Paris. Ben is establishing a career in singing in Paris and has important work to do. Karl wants to leave. But when Reinhardt turns up with news he is on the trail of his lover's killer, Karl helps him. Planning the killer's punishment is a lot easier to plan than implement. The trio's escape plans are threatened and Karl has to make a decision: kill or be killed.