All politicians are liars, Putin is no exception. It's just a question of making it look like he's the bad guy fighting the good guys. There are no good guys in politics.
There are number of parties that could have done this and Russia/Putin are the least likely for obvious reasons. With sanctions in place, the investigation regarding suspected involvement in US elections and only a few months away from the World Cup being held in Russia maybe it's not a good idea to risk getting involved in an international scandal on top of it, with absolutely nothing to gain and a whole lot to lose in a process. Well, who cares about all that, let's just make a poorly prepared attempt at poisoning our ex-spy who we pardoned and swapped years ago and who we haven't had use for in years in the middle of England with a nerve agent that was developed, surprise, surprise, in Soviet Union and pray and hope they won't guess it was us. Wow. Things must be really bad at Russian intelligence if that's how they go about their business. How did they manage to manipulate US electorate to vote for Trump when they employ such idiots, one wonders. Just how stupid do you think Putin is? Whenever I ask for motives, I keep hearing about bad reputation, oh he's crazy lunatic, dictator, he doesn't care about consequences, etc. Not a single logical explanation as to why try to kill this guy, why use this particular nerve agent and why do it in England of all places, and why now, of all times.
Unless of course it's done to hurt Russians, there's simply no other logical explanation for that, if people try to keep their bias out of the equation. It reminds me of Assad and how every time Syrian Army with Russian air force support is making progress fighting the Al-Qaeda jihadists, I beg you pardon, I meant freedom fighters, the news of yet another suspected 'chemical attack by Assad forces on civilian population' pops up all over the western media. Must be just a coincidence.