I think I just need to get this off my chest and hopefully get support and reassurance off other parents who have experienced this guilt.
I have a 3.5 year old son who although had his challenging moments is in lots of other ways a fantastic child and I love him to pieces.
I also have a 9 week old baby and my 3y/o has had to come to terms with a lot of adjustments since the baby came along. Me and DH expected a deterioration in DS's behaviour and agreed that we'd pick our battles with him and perhaps be a bit more lenient with him to accommodate his reaction to the baby arriving. In general that's worked well, he gushes over the baby, shows no jealousy towards him at all but me and DH are having to deal with some difficult attention seeking behaviour.
Today our 3y/o has been pushing our buttons and deliberately and repeatedly doing things we've asked him not to over and over again. This includes things like jumping on the sofa, throwing the baby's playmat around, trying to climb into the baby's bouncy chair and many other behaviours. I hate the way he looks at us when he does these things because it's like he's goading us. I know it's all part of him wanting our attention since the baby arrives but it's just so hard to manage sometimes.
It all came to a head this evening when after all day of him misbehaving he screamed when his dinner was put in from of him, refused to eat it (he's had it plenty of times before), went hysterical, started shouting, kept getting up from the table and trying to run off and as a result of him persistently being silly he ultimately knocked his plate off the table which ended up face down on our new carpet. He stopped in his tracks then because he knew things had gone too far. My DH told him that he wasn't getting anything else to eat and sent him to his room to calm down.
I was then in the living room cleaning up DS's toys and the baby was on the play mat in the corner of the room. DS suddenly ran into the room like a little whirlwind and went over to the playmat and completely unexpectedly started hitting the baby on his stomach which then led to the baby crying. I couldn't believe what I was seeing as DS had never shown any worrying behaviours towards the baby and I just lost it.
I put DS on the naughty step and told him that he must never EVER hit his baby brother and that I was extremely angry at him. I used a tone of voice with him that I've never used before but after him being disobedient all day and then hitting the baby I just couldn't take anymore. He messed around on his naughty step, shouting out, blowing raspberries and just generally trying to get a reaction out of us.
By this point me and DH were drained and told him that he'd behaved so badly all day that he was going to bed early.
Prior to putting him into bed I told him again how disappointed I was that he'd hit his baby behaviour and even more upset because he hadn't apologised for it.
Tonight DS was put to bed by DH (I usually do it) without having had any tea and no bedtime stories and songs which is what I normally do with him.
There's no way I could have done our normal bedtime routine though because I couldn't have him thanking that his awful behaviour during the day was being excused or that him hitting the baby didn't have consequences. I felt angry towards him.
Anyhow, I've just gone into his room and thought how angelic he looks when he's asleep. I tucked him in and kissed his head and the guilt just started creeping up on me.
His behaviour has never been as bad as it was today, usually he has a few little moments during the day but is otherwise well behaved. Today he was just so deliberately naughty almost from the moment he woke up and me and DH hit the point of feeling like he should be punished. I really hate that word. 
I'm sorry this post is so long, I just needed to get it off my chest. I have told DH that I feel guilty about how the day ended but he said I shouldn't as DS's behaviour had been awful all day and that hitting the baby just cannot be condoned.
Has anyone else had days and feelings like this?