I don't know if this helps at all, but you're describing my kids, just the other way round! DS1 was always calm and placid. He takes everything in his stride and is an all round "good egg". DS2, on the other hand, is nothing short of a fiend from hell!
But let's look at where you are. He's 3. He's done the "terrible twos", they're out of his system. What you then have is what my friend Sue and I have coined the "teenage threes", which is like the terrible twos but with an opinion about everything. And only their opinion counts. Anything you say is automatically wrong or in some way designed deliberately to ruin their day. In other words, just like a teenager. If you add that to the frustration of being behind with his speech, you have a recipe for disaster. In fact, if it weren't for the speech delay, I'd say it was probably jealousy over the baby.
So about the speech. And this is the tricky bit to say. Has his hearing been tested to rule out hearing difficulties which could account for speech delay? If so, and considering he seems outwardly sociable so you wouldn't automatically assume anything like autism, have a look at
http://www.pdacontact.org.uk/frames/index.html. Children never behave inappropriately for no reason. There is always something underlying their behaviour. And whilst I know it's horrible to contemplate labels, sometimes they're the only way to get the help and support you need.
I remember when my DS2 was about 3 I dumped him in his bed and told DH to "take that away!" I just couldn't take any more. We've been sent on so many parenting courses we could probably teach the blasted things, he's been assessed for ADHD because the school said that was definitely his problem. Not only was the school's assessed ADHD score lower than ours, he had a score that was lower than a "normal" kid would get. Conduct disorder, on the other hand, had a score so high that there wasn't a score for it on the chart. It sits proudly all by itself above the chart! I've lost count of the number of insults I've been subjected to ("can't you control him?", "he wouldn't do that if he was mine", "he'd have his hair cut / wear what he was told to wear etc if he was mine", "mine never behaved so dreadfully"). Strangely, when offered (with a completely serious face) a small boy, a packed suitcase and a golden opportunity to prove it, they quickly decline.
Over the years (ds2 is now 9), I've learned to adapt to him. I've even learned how to get him to do what I want some of the time (he's a typical bloke, so you have to get him to do what you want by making him think it was his idea in the first place

). And I've definitely learnt to deal with his meltdowns. Which is just as well because I still have problems with the shoulder he dislocated when I tried to restrain him during a meltdown in the middle of Sainsburys. He was 5. I love my two boys more than anything else in the whole world, and they know it. But I don't like the things he does. The trouble is, he has a problem. And until we can get the help he needs, he'll always have this problem. But we're getting there.
So in conclusion, no, you're not a monster. You're a mother under pressure doing the best you possibly can. Rant any time you like
Lots of love
Lynne
ps. I hope it hasn't come across that I'm implying your son needs labelling in any way. It was just a thought.