TL;DR: Understand why your son is behaving that way, help him deal with his feelings, and use consistent time outs that don't alienate him from the family.
So he had failed to notice the impact of his actions...
Clearly he had not failed to notice the impact of his actions; the first time, it startled his sister and brought you to him.
I feel for you. I had a terrible time with my first two kids being jealous of each other. But shouting does nothing good. It teaches the child that words are not enough, it makes you feel bad about your parenting, and it is a behavior that is likely to be repeated by him with his children. Though I can understand it, shouting is out unless his life is in danger (he's running towards the street.) Shouting should be reserved for emergencies, so it retains its impact.
If he was a good little boy before his sister began being a "real person", I suspect jealousy is at the core of this behavior. His sister is getting a lot of positive attention (the fun of learning how to walk, the growing independence, all the cute things they do at this age), while he probably gets less. He doesn't remember that you were just as thrilled at his milestone achievements.
So, empathy is in order, from you and from him. Pay a bit more attention to him. Don't just praise him (that leads to problems itself); praise real efforts when you see them, e.g.:
You're playing well with your sister. It's hard to play well with a baby; they aren't interested in the same things you are. Great job!
I see you're working hard at building that Lego set. It takes effort not to give up when something is difficult. That's great!
Empathy is teachable. Read children's books about sibling jealousy. Make sure he has an emotional vocabulary ("I feel ignored/unimportant/discouraged...) and the right to use it without retribution (his feelings are real, whether they are kind or not), and practice it yourself when you discuss things with him.
I feel discouraged when you roar at your sister, because I love you both, and I want you to get along. I also feel upset, because she is not happy when you do that. I want you both to be happy, and I want her to feel safe.
You might feel silly having a discussion like this, but it's better for both of you than shouting, which only makes your son feel unsafe.
Boundaries are in order as well. He's not allowed to roar, scare, hit or otherwise hurt his sister. Use time outs consistently. Time outs aren't under a child's control ("think about what you have done, and come back down when your ready to behave"); if he could control himself, he wouldn't have misbehaved in the first place. Time outs are limited. Five minutes. Too long, and the child feels banished from the family, and "forgets" why he's there in the first place.
To use time outs effectively, I recommend 1-2-3 Magic. The biggest problem that I hear about is that it's not used properly. Read the entire book before you try to implement it, and go by the book. It's effective when used properly, and you never have to raise your voice. Whether you warn first depends on the severity of the offense.
[Son], please don't take your sister's toys from her. Let her play with her things. She'll move on to another toy soon and you can play with it then." (He continues to take things away from her.) [Son], I asked you not to take your sister's toys away from her. That's a one.
or
[Son], we've talked about how you're not allowed to scare your sister. You just roared at her and scared her. That's a one.
Sometimes you go straight to three. Those are reserved for significant boundary violations.
Hitting is absolutely not allowed in this house. That's a three. Time out, now!
When they come out, you have a discussion about why they behaved that way (encourage them to express their feelings truthfully and respect their honesty ("Jealousy is hard. We all feel jealous from time to time. But it's important not to give in to it.") Explain why you discipline them (you want a good life for them, and self-control is important for that) and reaffirm your love for them. Your love is not conditional on their good behavior. But peace and harmony in the house are dependent on everyone's good behavior, and it's a win-win situation when everyone follows the same rules.
I was raised (fearfully) in a house of angry shouters (and hitters). I promised myself I would never scare my children like that. My pediatrician gave me a copy of 1-2-3 Magic on our first well child visit, and I am so grateful that he did. It worked like, well, magic. The kids always understood why they were being disciplined, and I almost never got angry, which I'm pretty sure would have been my default position, had I not learned a better one. It would have been much harder to find a better one than that, though some parents will differ with me. :)
