Hello,
This may be stupid, tell me if this is stupid, but this is what I thought:
free Malwarebytes case:
1) I download a clean software
2) I scan it on demand with free Malwarebytes, result: software is clean.
3) clean software was in fact evil and start downloading trojans 1 hour after scan.
4) oopsy computer is dead
free avast case:
1) I download a clean software
2) It is scanned by realtime protection, result: software is clean.
3) clean software was in fact evil and start downloading trojans, realtime protection prevents it.
4) yay computer is happy and well.