Dude, everyone who has spent any time with kids has definitely had a moment they are not proud of. Kids will challenge everything you thought you knew about life including your own ability to keep cool under stress. My thoughts on the matter, since others have already told you not to be too hard on yourself, are some tangible actions you can take:
1. Unlearn that response and replace it with something else.
Unlearn that rage response and spend time learning responses that you feel more proud of and that fit more with the person and dad you want to be.
If that was how you were spoken to as a kid or how someone "taught" you how to behave when you were young, you might need to do some work around this. Get support- there's a lot of great stuff out there (therapy, dads groups, etc).
2. Your daughter deserves an apology too.
Not to pile onto your bad feelings, but please consider your daughter in that moment too... she was looking to you for calm and comfort to help her with her stressful situation and then you became enraged and raised the stress level. Whether yelling at the boy was justified or not isn't even the issue in her case; when you became overwhelmed with your own emotions before being curious about or consoling to hers, it amounts to you shutting her down and ignoring her when she asked for help. Not to mention you becoming a danger to her friend and how scary that might have been for her. She had to shut down her feelings in order to explain to you that it was an accident and get you to calm down. All that to say, you might consider apologizing to her and talking through the situation and your mistakes with her too.
For all the fathers of daughters who commented: your daughter will stop telling you things and stop coming to you for help if she can't trust your response to be helpful. Practice now so she'll come to you for more serious situations when she's older.
3. You're doing fucking fantastic.
Your awareness and questioning means you're on the right track. Keep asking! Keep growing and learning and you'll stay a good enough dad and have a great relationship with your kid into adulthood.