Why Lamplighter Kill Himself?

Lamplighter's suicide is a brutal reminder of his moral allegiance. After being apprehended by The Boys, Lamplighter went along with Neuman's plan for the hearing and likely would've gone ahead and testified had he lived long enough, but he never actually sided against Vought.

Why was Lamplighter kicked out of the 7?

Lamplighter was originally a member of Vought's premiere superhero team The Seven, but had retired just prior to the first episode. ... In reality, Lamplighter likely didn't retire but was forced off the team due to killing CIA agent Grace Mallory's (Laila Robins) grandchildren.

Why did Lamplighter kill Mallory's grandchildren?

Through flashbacks, we learn the Boys blackmailed Lamplighter to spy on the rest of the Seven. This made him incredibly angry, which is why he went after Grace's grandchildren. However, Lamplighter later reveals that his intended target was Grace — killing her grandchildren was an accident.