The people who are paying you to build the land mine have interests that are different from yours -- perhaps that's not true at the moment, but it almost certainly will be in the future. They are very likely to sell your handiwork to someone else, perhaps under financial or legal duress. This may happen several times before the explosives in your creation decay into harmlessness. Chances are, any influence you may have when you manufacture your mine will only diminish with time. It is not only possible but likely that what you are building will fall into the hands of people whose interests and values are far removed from your own.

Of course, it's unlikely that you will be the one who eventually steps on your own mine. Whether that makes a difference to you depends on -- and speaks volumes about -- your character as an engineer.

Of course it answers your question. The answer is that there is no answer. You simply have no way of knowing whether a given patent will be used defensively or offensively, ethically or unethically. Like any good monster it has a life of its own.