ADT security is misleading

In 2015 ADT advertised free alarm installation, I took the package and have have since being making monthly payments. On the day of installation the agent told me to pay R411 and later same day said that I must pay R850, the amount which I paid to him. A year later credit collection company called me since then to now saying that i must pay R411 for alarm installation by React company which installed the alarm on behalf of ADT. I indicated that I will not pay the amount since I have already paid R850 even though ADT said the installation was free. I have contacted ADT on this matter and all i hear is that the matter has been elevated while I am getting calls from collection agents. ADT agent did confirm that the person who installed the alarm was not honest and took the money. My question is why did ADT bring into my house a person who is not trustworthy, could this be the same person who will come to attached my family since he knows how the system works. ADT is deceiving people by saying they are running specials only come later to demand payment for the same. In my case even though I have paid even more than what they quoted. Should I trust ADT with my security? I will be cancelling my contract with them for surely I cannot afford to pay monthly premiums for 2 years and yet fail to pay R411.