Is they are free of malware/backdoor

[DucooA]

Is they are free of backdoors?

[DucooA]

And why the zip passwords are "infected"

[Cryakl]

Is they are free of backdoors?

Yes unless specified otherwise in the readme, and the backdoors are in the stub/output only. (IE: WizWorm)
The password is 'infected' because they are malware samples, it's for research/testing only.

[JohnDoePang]

Is they are free of backdoors?

Obviously, these tools are collected from random places on the internet. If you use them to hack others, the backdoors in these tools will compromise your virtual machine, so attackers can pivot and damage or harm the people you've hacked, all this because of you. Some backdoors can stay idle for days or weeks encrypted within the tool to evade your suspicion. Or the tool can download the final payload after idling for long time.

[JohnDoePang]

And why the zip passwords are "infected"

Your antivirus will obviously delete all these tools immediately after you download them coz they are RATs, bruh, grayhat tools on the fence of being used for good and bad.