You can propose anything - the DM role plays your patron's response
As this edition of the game is built, the DM can do what you described.
The source of pact magic powers are indeed the patron, as you noted. (PHB, Warlocks, p. 105-108). What you may have forgotten to take into account is that a contract (a pact) is a two-way negotiation with both an offer and an acceptance - and a bundle of conditional statements that both parties finally agree to.
In contractual terms: your proposed contractual promise was your offer of (this many dead per this time period, conditional upon character level) but your patron seems to have rejected that offer. The DM is playing the role of every NPC and deity, and every other circumstance in the universe (DMG, NPC) - you and the other players play the PCs. The DM can indeed role play it that way, or rule it that way.
What do I do now? (In Character)
Re-open negotiations with your patron (i.e, with the DM) and see if you can work out a deal that is acceptable to both characters: your Player Character, and the Patron (who is a Non Player Character - in the strictest sense - played by the DM).
What do I do now (Out of Character)
Experience based point, FYI: character generation works best as a collaborative effort between the player and the DM. If your discussion begins with the tone you presented here
Was he right to shoot me down like that, or is it within my rights to customize my contract however I want?
then I suggest you take the time to have a one-on-one discussion with your DM rather than calling them out. I am confident that you'll both work something out that fits the group you are playing with.
Something to think about, given the nature of the pact idea you had - which certainly fits with both the Raven Queen and a sentient blade, the paragon of which was Elric of Melnibone's Stormbringer: what if the DM's internal reaction (which wasn't spelled out point blank to you at the table) was something like this? I've no idea what the DM's thinking was, but let's look at this from a DM's point of view: a pact that required you to become a walking abattoir might not have been something that fit into his world's cosmos. Anyway, have a chat and work it out. And have fun.