Theory:

GERRARD: You mean to live with me? A trifle sudden isn’t it; you’ve not been invited.
 
INTRUDER: You won’t be here long; so I didn’t trouble to ask.
 
GERRARD: What do you mean?
 
INTRUDER: This is your big surprise. I’m going to kill you.
 
GERRARD: A little harsh, isn’t it?
 
INTRUDER: (with heavy sarcasm) Yeah, I’ll be sorry to do it. I’ve taken a fancy to you, but it’s just got to be done.
 
GERRARD: Why add murder to your other crimes? It’s a grave step you’re taking.
Explanation:
 
Gerrard was confused when the intruder informed him he would stay at his (Gerrard's) cottage. He asked the intruder if he wanted to join Gerrard and told him that his choice was unexpected. Gerrard had pointed out to him that the intruder had not been asked to stay at his cottage. The intruder answered that he didn't bother to seek Gerrard's permission because he (Gerrard) wouldn't be around for long. Gerrard was shocked to hear those words, "You won’t be here long" and asked the intruder as to what he was alluding to.
 
The intruder had already informed Gerrard that the cause for the intruder's entry into his cottage would not be a pleasant surprise. The intruder then revealed why he had broken into Gerrard's house: he wanted to murder him. The intruder informs him that it was the major surprise he was going to deliver. Gerrard told the intruder that he sounded very harsh. The intruder sarcastically answered that he would be sad to do so since he had gotten fond of him. Gerrard advised him not to commit murder because it was a severe crime, unlike the jewel robbery he had committed previously.
 
Meaning of difficult words:
  
S.No
Words
Meaning
1.
Trifle Slightly  
2.
FancyTo suddenly start to like someone
3.
SarcasmThe use of remarks which means the opposite of what someone says
4.
GraveSeriously bad
Reference:
National Council of Educational Research and Training (2006). Beehive. If I Were You - Doughlas James (pp. 138-144). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.