It's rare, but it happens: some clients on Fiverr either start out abusive, or become abusive down the road, for whatever reason (a misunderstanding, or their own client pressures them, or cultural differences, or...). You might be afraid of a negative review, but you're not obliged to tolerate abuse, and in most cases, as long as you remain calm and professional, you will get help.
So, what to do if your client is abusive?
1. Try to Communicate
If the situation doesn't cross a certain line (for example, the client tells you that he/she wishes you were dead), it could be a misunderstanding, so politely explaining what's going on from your point of view might help. Remind the client what they requested of you, and what, exactly, you did to fulfill that request. Remind them that you want them to be happy with your service, and ask them to explain why they're not satisfied. It could have been just the language barrier and/or cultural differences, so this approach is definitely worth a shot.
2. Stay Polite and Professional
No matter how abusive the client becomes, remember to stay polite and professional. If necessary, step away, calm down, and then respond to the client. Harassment is strictly forbidden by Fiverr's Terms of Service; neither you nor the client are allowed to break the ToS, and if your response is unprofessional, you could get yourself in trouble. Besides, if someone is abusive on purpose, nothing will anger them more than seeing that they're not affecting you.
3. Document Everything
Make screenshots of the entire conversations, both in the inbox and on the order page. That way, if the abusive client doesn't quickly calm down, you have all the proof you need to support your case.
4. Contact Customer Support
If the client refuses to behave reasonably, and keeps abusing/threatening you, submit a ticket to Customer Support, briefly and politely explain the situation, send them the screenshots, and ask for their help and advice. Keep in mind that, due to the Privacy Policy, they can't tell you what they did with the abusive client (other than telling you that they have removed the negative feedback, if you have received it).
5. Be Patient
Sometimes Customer Support is surprisingly fast. Sometimes it might take them time to get back to you (say, they have a ton of tickets to respond to, so you have to wait for your turn). Don't open multiple tickets (it will just slow the whole thing down), patiently wait for the Customer Support to answer you, and once they do, don't forget to thank them for their help. If you feel that they didn't help you and if you disagree with their decision, it's still a good idea to stay polite; kindness and politeness will always get you much further than "yelling" at someone already swamped with work and complaints.
In short, stay calm, polite, and professional, don't break any rules yourself, and in most cases, you will receive the help you need.