Raising Rent Issue

[Cory St. Esprit]

I just closed on my first small apartment building (4 units). We have some commercial and international real estate we rent out, so now new to the rental industry - just apartments. Seller had the apartments significantly below market value (tenant paying $500 on an apartment that comped at $750, another paying $625 on an apartment that comped at $850). Leases were month by month, so I had my attorney draft a new lease that increased rent by $25/month over the next ~12 months to get apartments up to market rate - and the one long term tenant (12 years) decided to leave in 30 days. 

Honestly, this tenant has a cat that isn't approved, is doing drugs in the house (I've seen it with my own eyes), and keeping the place dirty...do I call it a blessing because they're leaving and I can fix up the place to rent closer to market...or did I do something wrong?