When Lazarus was a child during the London Blitz. He was horrifed of all the death it caused, and for the rest of his life seeked ways to avoid death.
When he finally manages to build a machine that can reverse the ageing progress, he shows it off in a public. Lazarus steps inside the machine to show it off, but it goes haywire and the Doctor shuts it down. When Lazarus steps out looking younger, the machine is deemed a success. But the machine activated dormant genes in his DNA, this causes him to uncontrollably transform into a evolutionary throwback which can suck up the life energy of humans.
The beast chases the Doctor and Martha throughout the building and they hid in his machine. When Lazarus turns the machine on, the Doctor reverses the polarity which causes it to blast Lazarus. When he he taken away by an ambulance, he woke up and killed the medics inside. He escaped to Sothwark Cathedral closely followed by the Doctor. He transformed back into the monsters and chased Martha and Trish up to the bell tower. The Doctor amplifies the sound of the pipe organ and plays it. The noise made Lazarus fall to the floor below where he died from impact.
It later turns out that Lazarus had been used by the Doctor's archenemy, the Master, who planned to make the Doctor grow old with Lazarus' technology.