A very interesting situation, @Durga- it looks like you will be free from the negative energy of your co-worker for a while! Sorry you have to deal with that- no one should dread going to their workplace each day.
As for your question about whether your spellwork may have caused the situation, let’s take a look…
Different practices and traditions have varying options on casting circles, but many will agree that casting a circle is not a required step to do magick.
Same with drawing on the divine- while divine energies and the assistance of a God/Goddess can certainly enhance your spellwork, getting help from a deity is also not required in order to do magick.
So even if you did not cast a circle nor ask for help from a deity, the lack of these things does not mean your spell didn’t have any effect.
The most important part of magick is your intention. Spells will follow the direction you set for them- although it often happens that different intentions can yield the same outcome.
Whether you were casting with the clear intent of removing your co-worker or were wishing for justice or were asking for peace in your workplace- all of these paths could lead to the same outcome: that your co-worker is out of the picture, at least for the moment.
In this sense, it looks like the intention you had, paired with the mantra you cast, helped to yield the result you desired. I’d say that the spellwork was a success! 
As for the ethics of the matter? Well, ethics are very personal- they are something that each witch must explore and define for themselves. If you wish to follow ethical guidelines like the Wiccan Rule of Three or the Threefold Way or Karma, then in general, it’s recommended not to influence or change the opinions and actions of others.
In addition to the Spellcasting Ethics, I also recommend checking out Opinions on Black/Dark Magic and Taking Someone’s Free Will to explore different ways of outlining ethics in magickal practice
All in all, I’d say that it is pretty clear that you cast a spell (using intention and the mantra) and that is worked successfully- nicely done! 
As for setting your intention to be in line with whatever ethical path you choose- that is something you will need to explore and decide for yourself 
Blessed be! 