What I mean is : if I end the game killing most ppl using all the tools and gameplays for what they are built, will it prevent me to reach the "true" ending, and I will have to play again the full thing in "stealth" mode ?
It depends what you mean by "true ending".
There is a set of choices that is considered canon by the developers, and is reflected by certain assassination targets surviving and appearing in Dishonored 2. Most of these imply a low chaos Corvo (and Daud, in his DLCs).
Whether you get the "good" or one of the "bad" endings is decided at the beginning of the final mission, based on your Chaos, though it's possible to have such high/low chaos a few missions before then that you can't change it.
However, there's no reason you have to stick with those decisions. Nobody's going to ban you from playing Dishonored 2 just because you didn't do what they wanted. Enjoy your rampage.