"두번마음에 들었다가는 아주 그냥 사람 잡겠군"
== (그 여자가) 두 번 마음에 들었다가는 (그 사람은) 아주 그냥 사람 잡겠군.
=> If he takes a fancy (to her) one more time, who knows what (he) will do?
(The original sentence has both parenthesized parts omitted, so we don't know the gender of them - I assumed he and she)
It is a very informal and idiomatic sentence (with a lot of omissions) hard to translate properly to another language.
* 두 번 XX-다가는 YY-겠군 = if you/they XX twice, you/they might YY -> If you XX one more time, you might even YY.
It usually means the subject person has already acted once in a striking/unusual way in reaction to something (i.e. 마음에 들었다), so the speaker is left with the impression that if they go one step further the next time around, they might even do something as drastic as YY (사람 잡다).
* 마음에 들다 = take a liking/fancy to someone/something. -었다 indicates it has been done (present perfect).
* 그냥 = just; might even (do something); not stopping at anything.
* 사람 잡겠군 = 사람 잡다 is an idiom for "cause havoc on someone". 잡다 ("kill" - it's a different verb than "hold/grab/catch") in this sense means killing an animal (chicken, pig, etc) to cook them. When applied to people, it's an exaggeration for causing something bad (vaguely - it could be from a great mental shock to physical injury or even death depending on the situation). It is also unclear if the feared action is direct to themselves or to the other person, but I think it's the latter.