Poetic Justice?

I am a very literal person. I like to know what people are saying, and similarly I like to be understood. I rarely mince words, and like to know where people stand and I like them to know where I stand. To achieve that feat, one must be versed in the language and able to articulate yourself clearly. There is also a more subtle thing at play often in language and conversation, and that is assumption and intent. When you aren't clear about something, often times you assume that a person means something, when in fact they don't mean that all.
The reason for this rant, is that yesterday I was reading an article about an ATF agent who went undercover with the Hells Angels, and now fears for his life. A few years ago my book club read a book about a similar scenario however a different outlaw motorcycle gang was infiltrated-- the Mongols.
My dilemma is this-when is it OK to not follow a words true meaning, to lie and deceive and to digress from a words true intent. I realize what a naive questions this is-given the immorality of the gang members actions, etc. However, does that make what the undercover agents are doing right?
By going undercover, we are asking these outlaws to trust us (the system). Now, these folks are already a distrusting and skeptical group-so that isn't an easy request. Over time, the undercover agents build that trust brick by brick, slowing grow relationships, and one of the undercover guys actually reporting a feeling of kinship and true acceptance.
Then after we have infiltrated their minds, families, and connections using deceitful words and fake actions--then they are exposed and brought 'to justice'. I don't know what the meaning of justice is, but I can assure you that these gang members will never trust another sole for their entire lives. Additionally they will pass on the inability to trust, the skepticism of the 'system' and an overall disdain and (further) disregard for human life.
Now, again, I understand that the outlaw motorcycle gang members are criminals, often times violent criminals with actions such as rape, child abuse, and spousal abuse under their belts. However, they are still human beings.
While I feel bad for the ATF agent who now fears for his life...where was his common sense when he went undercover and saw what these criminals were capable of. Where was his long term planning for his kids, family and safety...? Hmm, sounds like he got burned the same way the outlaws did.
Since I don't feel that true justice is being served in a scenario like this, perhaps this is what they call poetic justice? You decide.