Petr Levkovskiy, 42, heard a commotion outside his Bull Creek home on Sunday morning and offered to track down a child’s stolen bike.
“It’s quite a sad circumstances that have occurred,” said Detective Superintendent Leo Ricciardi.
“You’ve got a basically a good Samaritan trying to help some children when he’s had his pushbike stolen and you wouldn’t think that this would be the end result.”
Police say he and his wife approached the alleged thief, a 15-year-old boy, about 10.20am on Sunday.
“He approach that male and confronted him,” Superintendent Ricciardi said.
“An altercation took place and as a result, the 42-year-old male was stabbed in the stomach.”
Lefkovsky was taken to Royal Perth Hospital but could not be saved. The teen was taken into custody on Monday afternoon.
The slain engineer had moved his family from Russia just a few years ago because he loved Australians.
Friends described him as a joy to all who knew him.
“He was just being the kind, good-hearted Samaritan he always was,” one said, on Facebook.
“Petr never even thought before jumping in and helping someone else, it was simply second nature to him.
“Nothing was too much trouble. He was one of the most gentle and sweetest people it was my privilege to know and we will all miss him.”