Vampyr does not want you to “save early and often” in order to find your optimal path through the game. Dontnod representatives confirmed that they’ve taken save controls out of the hands of the player to emphasize the importance and power of your choices.
As a vampire, if you feed on someone, you get a huge pile of experience at the expense of that character dying. This power boost is enough to trivialize the enemies you’re “supposed” to be fighting, and you can now stand a good chance against enemies whose main purpose is to discourage you from exploring advanced areas yet. Do this too much and you’d completely break the game, so they created a counterbalancing mechanism called “district health”.
District health is a sliding scale that evaluates and sums up factors like the health of the characters in that district, the status of quests that can be completed there, and the circumstances in that area as a consequence of the narrative. If district health is good, the merchants there will give you a discount on their wares. If it’s bad, things get more expensive. If it ever falls below 50%, the district is considered lost to chaos; all the friendly/neutral NPCs die, and tough enemies stalk the streets. Drinking folks for their XPs is a good way to damage district health: if you’re playing this to be a badass vampire with monstrous superpowers, that will eventually come at the expense of conversation and investigation, the more humanitarian aspects of the game mechanics.
While the game does briefly warn you that allowing a district to fall will kill all the conversation partners native to that region, some choices have a bit less transparent outcomes. I’m blurring this text, but in light of the Vampyr policy toward save files, I wish I’d known this in advance:
At the end of your first major trip to Whitechapel, Nurse Crane is revealed as the blackmailer you’ve been commissioned to stop. Her motives are arguably just, and your sponsor has requested discretion and a lack of violence. She asks if you’re going to go to the authorities, but her tone is one of righteous indignation. Your options are “[SPARE] No, but resign from your position with the hospital,” “[KILL] Enough of this, I’ve decided you’re more useful to me as a comestible,” and “[CHARM] You will forget what you have discovered. Your undercover days are behind you.” Complicating this is the fact that the CHARM option has an opened lock icon next to it, suggesting this option is one which has been unlocked somehow, so I picked that one. Subsequent events and some reading in the Steam community reveals that this is every bit as lethal to her character as killing her outright would have been, as she is transformed into a monstrous creature to fight on your next visit to her home.
What I’m trying to get at with these examples is that Vampyr goes to a lot of efforts to provide both in-game and metagame reasons to be enticed by your vampiric powers, and it provides both in-game and metagame reasons to regret succumbing to those temptations. I like it a lot, but I’m still gonna restart my game.