Fundraising (business): Is it considered OK for an angel to conduct a few months of conversations with the founders, during which a certain company valuation is discussed, and then finally provide a term sheet that is 50% of what was discussed?

The term "angel investor" is quite misleading, as it implies that the investor is benign in nature, one of the good guys. This is a ruse. All investors,  except maybe your immediate family, are motivated by profit, and profit alone. They may claim otherwise, but that just makes them liars. Ethics don't really come into it beyond whatever laws and regulations are relevant to your situation.  If you expect your "angel" to hand over a single penny more than they absolutely have to, out if the goodness of their hearts or some moral obligation,  then fuhgeddaboudit. They will give you exactly what is needed to close the deal. If they find a buyer for their shares, they will sell if it's good business. Even if it means that your startup will be merged, asset-stripped or restructured into oblivion,  they will take that deal if it is the most profitable exit. Trust investors at your own peril. In this case... Do you have options? Can you politely (always politely! Emotions have no place in negotiations) decline their offer and pursue other investors, alternative funding or even go it alone? Then point this out to them, and be prepared to walk away if you don't like their reply. But if you have to take their offer (or shut down), and that offer is just enough to reach the ipo (or whatever the investor's exit point is) in decent shape, then they have done a good job and you have been out-businessed. Remember that business is war, and that while everyone claims to be in it for the glory, only the idiots actually mean that. All the non-idiots are in it for the spoils, and they never take risks they can afford to avoid. The most successful investors hardly take any risks, because they can afford not to, and still profit. Don't expect someone like that to play nice and be a nice sport. They certainly don't expect you to.

I would strongly caution you against mixing social/emotional with economic discussions.  Talking about "rudeness" with respect to an offer has absolutely no bearing on the offer.  Here is a way to think about it: 1. Is the offer itself attractive, meaning does it represent a deal you are willing to take, and is superior to other offers that you have available to you.  If it does not, then negotiate - find an alternative, hold out, there are a million ways to try to improve your position.  And understand that they may be what you get. 2. Do you want to work with the people?  This is much harder.  If they are just trying to "jam" you, then you should assume they will jam you in the future.  If it is your only option, then it is your only option.  However, you should feel safe presuming that when they have the advantage in the future, they will capitalize, and you should be prepared to do it back. Don't mix the two questions. Term sheets are not rude any more than stock tickers are rude. Behavior is human. Make sure you pick well the people you partner with.