"I don't like a thief"
Even though it looks really different, I think it's related to (ku)iba 'to steal' ... usually to indicate the human agent of an action, you add the m(w)- before the verb stem and something like -ji or -zi afterwards, with a consonant sometimes being dropped, affecting which ending it takes (like (ku)andika >> mwandishi) so this just looks like that process.
Wow, this is so impressive! And yes, definitely I would like to understand all these processes in the language. Sometimes we can see also the Proto-Bantu root or other related forms as Zulu in Wiktionary entries. If only it would be necessary to read a grammar text about Swahili but I think it is good to begin the understanding of this point as you are offering here and then we can link it to other languages. I was just listening to Xhosa songs and not only to hear the clicks (the clicks sounds are really interesting in this language), but also because there are some forms of words that I can hear to be very similar to Swahili forms, for example the word wako. I really do not know if it has the same meaning in Swahili but perhaps I can start some courses in Memrise or other site to know this. I think this learning can be more complicated but sometimes it can be really helpful. Thanks a lot for adding this interesting aspect of the language. :)
Why only one thief ? Sipendi wezi - I don't like thiefs - would be better.