Think back to how you felt after 9/11. It's meant to be a similar sort of moment.
I was surprised that nobody had flown a passenger plane into a sky-scraper sooner, I remember contemplating what would happen if somebody flew a jet into the Empire State Building a few months before.
I guess the news of terrorism doesn't shock me. As
Jimmy Bob said, I grew up in Ireland through the 80s and 90s, and while I lived far enough away from Northern Ireland that I never encountered terrorism personally, it was constantly in the news. I remember where I was in 1996 when I heard that the Provisional IRA abandoned their ceasefire and bombed Canary Wharf, and the day in 1998 when the splinter group Real IRA bombed Omagh, but I was too young to really care.
So when September 11th happened I thought that the scale of the devastation was impressive and I was saddened by the loss of innocent life, but I certainly didn't think that everything was different.
I think part of the problem is JMS expects viewers to "live vicariously" through the characters. As you said, Santiago hasn't really been seen, so you have put yourself in the characters' shoes and try to think how you'd feel if your President had been killed and you're the only one who knows it was a murder conspiracy.
I'd wonder why anyone would go to the effort to kill someone with such minor powers. But if somebody killed my Taoiseach... I still wouldn't care that much.

I'd be shocked and I'd want to know why it happened, but at the end of the day some new guy is going to be put in charge and things will continue as usual. I guess I'm just too cynical to think that the death of a politician will change things that much.
Ah. Well, that's what the Location field is (rarely) used for.
You mean my alcoholism and extreme love of potatoes didn't give it away?
I used to use the location field of other sites but found that random American people I didn't know would be shocked by my presence (because electricity shouldn't exist here) and on some occasions I was asked if I knew their far-flung relatives in Ireland. "Yes, I do know the Kelly family, I know five of them, but the chances of me knowing
your Kelly family is very low.

"
Luckily this site doesn't seem to contain the same level of imbeciles, but since nobody else uses the location field properly I feel free not to.
Keep in mind that unlike some modern arc shows such as "Lost" or "Prison Break", B5's main purpose is not to shock and surprise its viewers, but to tell them a story. Shocking plot twists usually work only the first time you watch a certain show, while a good story can make you rewatch it numerous times and still find it interesting.
But I kept being told by people that the finale to season 1 had a shocking twist and that it would really get me interested in the show. It might have had that effect on me had I not heard about the Shadows in advance.