Deactivating target change with mouse

[Nekommander]

I'm playing with keyboard and mouse, and it's working perfectly fine except for targeting. Whenever I lock on a target and move left or right (trying to intuitively move the camera to get a view of the surrounding enemies) it changes target. I have died in so many boss+minion fights because of that. Is there a method or command to disable mouse movement while locked on like there was in DS?

P.S.: Don't tell me to buy a controller. If I wanted to, I wouldn't have asked this question.

[Alexoferith]

I presume that this is your first playthrough. I spent over 800 hours in this game with keyboard and mouse. First of all, yes, you can change the setting from using mouse to other keys but unfortunately, the change of target is bound with change camera angle or looking direction. So, my advice to you is to DON'T look around when you have locked onto a target. My second advice is DON'T rush in to a fight with more than two opponents, especially when one of them is a mini-boss or main boss. Oriental martial art, from history, it is based on honour and hence always stressed on fighting on 1 to 1. But in battle, it is totally different matter. However, shinobi is different class. They have their own honour but their fighting art is sneaky from the start. I am sorry. I digressed so much. As a shinobi, you should eliminate the minions first by stealth.
In this game, your combat art only works on target that you have locked on. Only whirlwind slash is designed to deal with a group of enemies.
In Sekiro, in combat, always, well, it is advisable just to face max of 2 enemies and they are both in front of you. If you want to engage both of them, DO NOT lock on to anyone of them. The fighting mechanics work in such a way that you only have to face the enemy when parrying or blocking. So, if 2 attacks you, just move your mouse so that your Sekiro is facing the one that you intend to parry. So if the right attack you first, turn to him, parry, then move your mouse to the left to face the 2nd enemy and parry his attack, and so on. This is ok when facing minions but higher level enemies, if you mis-timed one parry, you are almost certainly dead as a result due to the attack from 2nd enemy. Well, that is me. I am not that good. I have seen FightinCowboy on Youtube facing 2 red coats. He was hurt badly though. He came out victoriously in the end but he was hurt badly. In fact, I think he was lucky that he managed to kill one of them first. It is the Castle Siege episode. Have a look. Ongbal, I had not seen him facing two high class enemies. And from his videos, he had loads of videos showing different ways to face a certain mini-boss or main boss, but he never made a video facing two or more high class enemies. Mind you, he only made video where he suffered NO damage at all. I am sure he can face two red coats but I think he knows that he will get hurt.
Anyway, yeah, don't look around while you lock on to a target. Always find a way to get one coming to attack you first. And never allow yourself to be sandwiched by two, one facing you and the other at your back, unless you are carrying out a deathblow to the one you are facing. Then due to the i-frame, while you are carrying out the deathblow, the one at your back, no matter what he does, you won't be touched. And the fighting mechanics is good but not that good where you can change target from the one facing you to the one at your back.
Finally, after spending over 800 hours in this game, I found that the fighting mechanics has never been designed for any move against the one at your back. I hope I make sense and the information is useful to you. Have fun and great game this is.
PS It is true. With a controller, because it has two joysticks, one for movement and one for looking. But I am just not used to it. I am a Last of Us fan. I bought Last of Us 2, I just could NOT get used the looking control. It was so frustrating that I had not gone past chapter one. I have been a PC gamer since I got my 286 years ago. This game is hard enough and I am still learning after over 800 hours. I would not advise you to get a controller if you don't want to.