As far as I see it, you have two choices: 'since' an adverb of time, or 'since' adverb of reason.
Choose adverb of time, then you must change 'I work' to 'I have been working here', as you started in the past, at the time indicated by 'since', and continue to do so.
Choose adverb of reason, then 'I work here' is the reason, and 'I have always been kind to everyone' is the corollary. I changed that to 'I try to be kind to everyone' because, one presumes, this show of kindness will be an ongoing thing. But maybe the implication is, that you've had enough of being Mr Nice Guy, and that the kindness days are over! In that case stick with 'I have always been kind to everyone' Depends on what you say next!