Was the Nat 1 too punishing in this case?

Hey, I just wanted to get some perspectives and thoughts on a situation in one of the D&D campaigns I'm currently playing.

Today the session was 4 hours of fighting in the defence of a castle. All of our characters where on the walls and shot down spells and arrows on the attackers.

In the third or fourth round my spellcaster tried to shoot Sorcerous Burst and I rolled a natural 1 on hit and also 1 dmg. The DM decided that my attack would hit the archer NPC next to me, who was full on health, and that the archer would fall off the wall and die instantly.

My character had never killed a human up to this point, in her backstory, and in the four to five months the campaign had been running. This was a shock to her, and she was completely stunned and overcome with guilt and despair. So she hid in the tower and later ran away. I basically waited two and a half hours for the fight and the session to end while playing out and explaining what my character was doing in the next 6 seconds.

Overall that whole situation felt extremely unsatisfying, random and a bit too punishing and story changing, given that her first kill was supposed to be a key plot point in her overarching story, and fitting options where planned together with my DM for not too far in the future.

So, what do you think? Was this too much of a nat 1 penalty, or was I simply too deep in character?

Edit: for clarity I'd like to add, that we always played with critical fumbles (even though I didn't know the word till now), but normally it was something like "your dagger falls to the ground" or "your griffon charges straight into the spear and is dealt 5 dmg" and not "you kill an innocent man". And I guess it was just kinda hard to play out that it happens to the one character in the party that never killed a humanoid before, since it was implemented in both her backstory and the campaign that she reacts badly to being forced to take part in harming people. So it felt like character consistancy for her to be instantly affected, when she kills an innocent out of accident. Even though I understand all comments saying it was too much and antisocial towards the group, since it was me who decided to play it that way. But there never was a big bad discussion or fallout on the table. I explained what happens and why, and that it felt a bit harsh and too much out of control, but never was there like bad blood or mood in the group.

I guess there was a lot for me to learn in all of this, so thanks for all the feedback!