The Old Bailey heard how Matthew Quesada, 26, reacted with rage when passer-by Alan Smith, 63, approached them at a cafe in east London.
The court heard Quesada went home with his daughter, collected a knife which he hid up his sleeve and returned to stab kindly Mr Smith five times in a “frenzied and silent” attack as he sat with family members to celebrate his daughter’s birthday.
Quesada, of Leyton, east London, was found guilty of murder and sentenced to life.
Judge Peter Rook told him yesterday: “You are a very dangerous young man. This was a truly grave crime.
“Your reaction was wholly unexpected and out of proportion. You over-reacted and took this as a slight on your parenting.”
Quesada’s partner, Maria Brigitte, 26, was found guilty of assisting an offender and given a nine-month jail sentence suspended for two years and 200 hours unpaid work.
Mr Smith was in the BB Cafe in Leyton on March 26 last year when Quesada entered with his three-year-old daughter who was crying.
After a few minutes, during which time she continued to sob, he asked if she was all right, prompting Quesada to reply: “What’s it to do with you? Go away.
“What’s it f****** got to do with you?”
Mr Smith was described in court as being “eager to avoid confrontation” and decided to move away from Quesada and went into the nearby Roma Cafe to meet his partner, daughter and son-in-law,
Meanwhile, Quesada took his daughter home to his partner Maria and upon entering the house found a knife which he then took with him as he left looking for Mr Smith.
The court was told Quesada did not utter a word as he followed him into the Roma cafe and attacked him, lunging at him across the table as family members desperately tried to intervene. The attack only ended when a chair was thrown at him.
The next day Quesada was found in a car with his mother Victoria Passley-Quesada, 54, having cut his hair. He also had his passport and details of flights to Brazil.
Roger Smart, prosecuting, said the horrific murder had devastated Mr Smith’s family.
His daughter Estelle Jennings felt guilt about his death and wished she had not gone to the cafe to celebrate her birthday.
His son Daniel Smith said in a statement: “He was the most important person in my life.
“My dad was like a guardian angel. I really miss him.”
Daniel’s brother Alex said their father was their “best friend”.
He added: “You could never meet a more loyal, generous and honest, amazing man.”
Patrick Upward, QC, for Quesada, said the killer had been abused as a child and over-reacted when his daughter was approached by Mr Smith.
Acting Detective Chief Inspector Steve Meechan said outside court: “Alan Smith was a well-meaning man who simply wanted to inquire about a distressed child. He paid for his concerns with his life.
“Not happy with verbally abusing Mr Smith, Quesada went on to carry out a frenzied and unprovoked attack on a defenceless man.
“His actions have left a family grieving for a loving father who had simply left his home for a meal.”