An angel of Death in Los Angeles.

A sociopath kills a young boy, a quiet teen, and then an
entire family before turning his attention upon Pavel Trusnik, an elderly
shut-in from across the street.

Loneliness can drive a normal man to madness, and Pavel
Trusnik is not a normal man. After committing a crime that leaves him shunned
and isolated, Pavel has only the fading memories of his tragically flawed life,
and his one great love. When the violent sociopath sets his murderous desires
upon Pavel, only an ancient order that knows Pavel's secrets can intervene. But
for the isolated old man, the vicious intruder is the only company he has had
in decades...

(Death's Order, Book
One)