I'm using Karabiner elements, an open-source program to remap the keys on my macbook air (running the latest Mac OSX High Sierra) and my apple wireless keyboard. It works very nicely.
However, I am concerned about the off chance that a malicious adversary can possibly use it to record keystrokes and passwords. This question was already asked on a Google Groups thread, and the creator of Karabiner answered. While there doesn't appear to be any reason to suspect that the creator of the program has anything other than good intentions, I would feel a lot more comfortable with a more independent assessment of this question.
Some further info: the Github page for the project has a decent number of contributors, and the creator of the program appears to be someone who I have no reason so far to suspect. However, I'm not as experienced in the world of open-source software development as some others here may be, so I was hoping someone with a better background can shed some light on this issue.