You don't attack the messenger, you attack the message, and you do it with better science. No, it's not OK to bring the fight to the gutter if an honest man is making an honest attempt to follow the science.
An ethical response to a challenge from the scientific community on the efficacy or safety of your product is never a win-at-all-costs response. When a scientist is researching your product, unless you can show the research itself is a fraud, the only acceptable response is to accept or further the research.
An ideal response is not only to pour funding into furthering the research, it's to work alongside the other team collaboratively in doing so.
The ethics don't change depending on whether atrazine is actually net harmful. The only way to determine if atrazine is net harmful is through research exactly like this. We need this kind of research to happen, or net harmful chemicals will remain in use. Remember, Tyrone Haze starting his research on atrazine because Syngenta asked him to. From that point forward he was performing a public service.
Hope this helps you understand why it's NOT acceptable for companies to behave like this.
A human would feel guilt. A corporation would feel nothing, except for the sound of money.
You're assuming that he's an honest man. He could be a dishonest man. He could be crazy. He could be persecuted, and still have a persecution complex.
I am slightly troubled by their use of Google keyword searches. I'll note that the Left didn't have those concerns with their 'smirking chimp' campaign.
Frankly, from the article Hayes appears to be either his own worst or second-worst enemy. One can either try to win a chess game or upend the board, but one can't choose to upend the board and then try to maintain one's dignity.
Edit: I forgot one important point. The items you mentioned overlook one very key issue: is atrazine actually net harmful? That's the only real question. If it is, then its makers have a duty to cease production; if it is not then they have a duty to continue. If it is net-positive, then it is _good_ for them to discredit someone who opposes it, since by definition his course of action will be net negative.