David Heiss, 21, stabbed Matthew Pyke to death in a "savage and sustained" attack at his victim's flat in Nottingham, the crown court in the central English city was told on Tuesday.
He then borrowed a pair of his victim's shoes and a baseball cap and flew home.
Heiss, from Limburg, near Frankfurt, had become obsessed with Pyke's girlfriend, Joanna Witton, jurors were told.
When she rebuffed his declarations of love, he hatched a plan to kill her boyfriend and disguise it as a suicide.
Heiss had met Witton online, on the warscentral.com site she ran with Pyke from their one-bedroom flat above a pub.
The court heard that he managed to obtain the address of her flat from a fellow game player and twice travelled to Britain, in June and July last year, to meet Witton face-to-face.
She and Pyke, her long-term boyfriend, allowed him to sleep in their flat before he returned to Germany, but Heiss became more persistent and the Britons became increasingly concerned by his behaviour.
In an online message, Pyke told the German: "I hate you because you decided to come into our lives. I hate you for ever laying eyes on the girl I have dedicated my life to. I hate you for your desperation."
The prosecution says that in September last year, Heiss flew to Britain and lay in wait outside Pyke's home.
After seeing his girlfriend leave for work, Heiss pounced on Pyke, plunging a knife into him 86 times, before changing his clothes and fleeing to Germany.
Prosecuting lawyer Shaun Smith told the court: "It's the prosecution case that Matthew had absolutely no chance at that door, where that blood is. He was stabbed straight away by this defendant."
Heiss denies murder. The trial is expected to last two weeks.