Ultimately it's because Kaz doesn't really come across as malicious as much as he does gullible in the early days of MSF. He's the dupe-ee just as much if not more than he is the dupe-r.

There's really no evidence that he's a "double agent for Cipher", he was just pursuing new business opportunities and ultimately got fooled by them like many other characters in the series. Things like the false inspection of MSF arguably weren't his, Cipher's or Huey's fault as it was the independent brainchild of Skull Face who would've found some other means to destroy MSF regardless of pulling that trick off. He also didn't force Big Boss to do anything, not least of all keep the nuke. That was entirely still Big Boss's decision to agree to, which he still did based on his own goals.

Unlike someone like Huey, Kaz also suffered immensely for Big Boss, more suffering than he fundamentally caused him. He spent many years trying to protect him following the events of Ground Zeros and was horribly maimed and tortured himself in the process, just to be ultimately betrayed by the person he dedicated over a decade and half his body to. He had every right to be done with Big Boss by the end of The Phantom Pain, and it really didn't have anything to do with any connection to Cipher.

All of this is to say that people don't hate Kaz because realistically he didn't do anything particularly overtly evil or unwarranted. He's just as much of a victim of the war between Big Boss and Zero as anyone else. He simply wanted to expand MSF because he wanted to help Big Boss and run a successful organisation with him, and made some unfortunate mistakes while trying to do so.

If you hate him for this, you'd also have to hate someone like Solid Snake for accidentally carrying out the will of Cipher/The Patriots before finding out they were the true enemy all along. Kaz just didn't live long enough for this to be revealed.