Most serial murder, with or without sexual connotations, is about power and control. It is not an easy concept for the majority of the population but it is the underlying motivation for many serial killers.
Happy jacks
to kill people and break the law
he was going to kill her and her eggs
He'd kill them if given the motive.
They were using him to get to Katniss and then they were planning to kill him.
Never to kill the arab
It was alleged that he was having an affair with a co-worker.
I do not believe that it will kill you(my dog chewed on one for a long time about a year ago and she is fine) however, I do not recommend that you do so
A by-motive is a hidden motive.
Jacks are for lifting or raising things. They can be screwed jacks or air jacks, or most commonly, hydraulic jacks.
Someone stepping on it or it getting run over by a car. Also a bunch of other things.
An ulterior motive is a motive that they have beyond the main motive. They often do not show that they have this motive and it is often not good. The ulterior motive is almost always has it for their own good only.