anytime you do even the smallest of work you risk the possiblity of needing a root canal, shame more dentists dont share this info before this type of treatment is given. whether it was this drs fault or was going to be is neither here nor there. you now are stuck with a tooth (or teeth) that are in need of more $$$ treatment. was the issue not having dental ins and paying out of pocket? there is a plan (well here in cali anyway) called care credit. you run an application for credit and when accepted you can get a "loan" to pay for treatment without interest for an amount of time depending on how much you need.
Yes a dentist can cause infection if instruments are not properly sterilized, which I do not believe is the case for you. Any kind of cavity near the gum line or near the nerve of the tooth may need a root canal. Sometimes, though, a filling can do the job. It may not be clear which is needed until after the doctor starts the root, OR until after you leave the office. Every person is different.
Think of it as you having a cold & me having a cold. We both go to the same doctor, we are both given the same medicine. I go back in a week even worse than I was to begin with & you are now fine. Did the doctor cause me to get more sick? No. Obvoisuly I needed a different treatment than you.
If you don't trust this doctor's judgement, get a second opinion. You are not obligated to continue any treatment with him. Do not, though, start then root canal & then decide you want to see a different doctor. It will end up more costly to you & insurance may not cover it.
yes i dentist can cause an infection