Can cracked software (or games) be classified as virus by Avast?
If malicious yes, and they often are. It is one of The most common ways to be infected. Those nice guys that create them and give them away for free often bundel it with a extra program they don’t tell you about
And using crack is considered stealing, many malware help forum give no help to users of cracked software if infected
Another question: Can cracked Minecraft launcher (not for minecraft.net) has a possibility to become a virus?
Yes.
I have another question, can some legitimate websites be mistakenly blocked by antivirus?
Yes.
Is there a website you have problems with?.. tell us and we can check it