There may be a difference between UK and American English. In the UK I've only heard it been used sarcastically, so you are being insincere. If I heard someone use it, I would assume they were angry with me.

Hi, this is most often used sarcastically, so yes I would say negatively, although it is possible to use it as a big thank you too.

I think the answer is that it depends who you are talking to. As Arthur and Sam have said, it is negative in the UK. Like Sam, I have only ever heard this used sarcastically by British English speakers. For example, if you do something which causes a lot of problems, embarrassment or annoyance for another person, that person would say "Thanks a bunch!" to mean "I really wish that you hadn't done that!". It seems to be used sincerely by AmE speakers, however. It's probably best avoided, in case you cause unintentional offence. It might be safer to go with a more unambiguous expression of gratitude.

Hey Ishtar, this sentence is usually sincere. It means "Thank you so much" and is usually just a nicer way to say "thank you". English speakers, however, are often sarcastic, so if someone's tone in speech is negative, they could use this sentence sarcastically. For example, if Sarah is walking and holding a coffee cup, but Tom accidentally runs into Sarah and spills coffee on their shirt, she might say, "Thanks a bunch, Tom!" And here, it's obviously not sincere gratitude, but more of a way for Sarah to let Tom know that she's upset, without being too rude. Let me know if you have any questions!

Nothing negative about it. It just means Thanks a lot