I just finished it as well, and while I disagree with R76 about Bianca – a "hopeless field agent" who happened to be the one who solved the entire mystery by herself, despite being intentionally blockaded by MI6 itself, and also the one who figured out his identity and location? – I completely agree that the show began to unravel in its second half.
It starts in an understandable "holy shit!" scene: the Jackal successfully shoots the German chancellor from a seemingly impossible 3.8-kilometer distance. Beforehand he takes several "test" shots to see how the wind is (literally) blowing, but then pulls it off thanks to being in a stiff sniper position. Every aspect was meticulous, and it worked. That would've also been the case for UDC had the extra shooter not shown up.
Except then he goes after UDC on his Croatian island, where he shoots him from over FOUR kilometers. On a boat bobbing in the sea (making it exponentially more difficult to hit anything, period). He misses AND somehow magically deduces the exact moment UDC will come up for air. Despite UDC's entire security team knowing he's there, apparently they had no plan along the lines of, "Swim in an unpredictable direction so the Jackal can't blow your brains out!"
If the German chancellor shooting was at least within the realm of the possible, a successful sniper shot – in the forehead FFS! – from a wobbling boat is not. Even more stupidly, how the actual fuck did he OUTRUN all the boats chasing him? What, UDC didn't have a helicopter on the island? And nobody has radar?!? All we see is him speeding away while being chased by several boats at around the same velocity but then .... cut to him back on the mainland. (And while I know the Jackal's a sniper, is there some reason he couldn't have just launched a torpedo towards UDC from the boat? He was in the army, but he'd still have the know-how for that, particularly considering he found someone who could custom-fabricate his sniper rifles.)
And to be clear, this Jackal is not a "psychopath" – and quite intentionally so. He's driven by rage from his time in Afghanistan, but a segue from "killing off his entire Army division because they loved shooting up women & children" to "contract killer for anyone & everyone" is a rather considerable stretch. (It's not like Germany had anything to do with his deployment.) It's also not really PTSD, at least not in the traditional sense. He could, however qualify as a sociopath at least in part: he feels remorse, especially when he had to kill the older British couple on holiday, but doesn't hesitate to kill anyone who gets in his way as quickly as possible. (Including poor Erasmus.)
The part of the show that makes no sense – and not just because it isn't in the book – is the entire Spanish element. While I get that it was a necessity to give him .... well, I guess feelings, given that it's a continuing series, his trademark meticulousness fails him at nearly every point. He builds a house with a much too easily found hidden room – and he KNOWS it's not that hard to find, hence him having cameras all over the place. He flies nonstop from Cádiz to Croatia for the hit, in disguise as an older man but still visibly Eddie Redmayne, but only AFTER – almost bizarrely – assassinating a Cuban (!) named Jimmie (!!) because his brother-in-law kinda-sorta made a deal with him and he's a gangster of some sort?!?
And finally, it ends on the dumbest, most predictable note of all: the Jackal going on the hunt for Nuria & Carlito. MI6 and every security service in the world now has his file, thanks to Bianca's work that got her killed.
Not sure I'll be back for a second season.