They mis-designed the fuses on the devices they were carrying thus fatally setting fire to themselves rather than anyone else (and thus causing their major explosive device to fail to detonate), failed to disrupt the functioning of the airport because they hadn't accounted for the fact their vehicle wouldn't fit between the bollards at the terminal, did virtually no harm to any innocent and one of them got beaten up by a Glaswegian baggage handler
while still on fire. That was about as 'poor execution' as whoever was steering the Titanic. If setting fire to yourself while getting a kicking from a mouthy Scotsman doesn't count as fail I don't know what does.
The fact is the last successful explosive device on a plane in Europe was Lockerbie. Since then several people have tried - Glasgow, Richard Reid etc - and almost always managed to harm themselves badly while at most inconveniencing a few other people. Now, it would be fair to say that can be partly attributed to the security we have at airports which is above & beyond anything we have at train stations and etc but that doesn't mean that more security will by definition be better.
There's a similar consideration in system engineering - you essentially have a continuum with 'security' at one and end and 'usability' at the other. The ideal point is somewhere between but this kind of tomfoolery doesn't contribute positively in any way.
Jon
