It sounds harsh on the internet, but it's a sound business decision based on risk factor.

@Alex R. didn't realize you were a newbie, apologies for the harshness, but  RE investing in Baltimore ain't no joke as you can see.

If you are too sensitive or I am to harsh, well that's a different story.Clean up, re rent, disclose that your rental is a murder scene (if required by law)....that's the wisdom.FYI, I have landlorded and lived in warzones, so I'm not judging doing either, just observing the apparent reality, again my apologies if that is to harsh or an incorrect assumption.

I tried multiple chemicals including Comet to remove it, but didn't worked.

Living in Canada with harsh winters is that something that could break a deal for anyone?

Most exterior cap ex items last longer in milder climates than harsh climates resulting in lower long term costs. 

Do you guys have your cleaning service take care of rhe cleaning and chemicals?

I have also asked her to send me information of what chemicals she used to bomb the unit as well as dates.

Cheaper options include using chemicals like Odoban or trying an ozone machine to clean the area.

You couldn't be bothered to use a 50-year-old weed control chemical on an overgrown yard because you believed some quack somewhere that told you it might cause cancer.Now you're hypothesizing that your dirty tenant brought a breeding population of rats with him.Come on, Peter.