Abdul Raqib Amin did impressions of the TV comedian Ali G when he got into trouble with teachers at school. It is claimed he asked them: "Is it cos I is black?"
The schoolboy turned extremist also annoyed teammates on the rugby field, attempting dangerous "clothesline" tackles and playing as though "he had a screw loose".
The revelations came as Alex Salmond said the Scottish authorities would take a "zero-tolerance approach" to anyone who encourages hate crime against the Muslim community.
The First Minister said the actions of Amin, who appeared in a recruitment video for the extremist Islamic group Isis, did not reflect mainstream opinion in Scotland.
At First Minister's Questions at Holyrood, the SNP leader said: "Radicalisation is something that we have been and continue to be vigilant about.
"Police Scotland has been very active in monitoring that, but also proactive in engaging with and building strong relationships with the Muslim community."
The former St Machar Academy and Sunnybank Primary schoolboy spent his early years in Scotland after moving from Bangladesh. During his time in the Granite City he played rugby with the Aberdeenshire Rugby Football Club, between 2006 and 2007, as a centre back.
One former ARFC teammate, who was also in his year at St Machar, said: "Whenever he used to get in trouble from a teacher at school he would say 'Is it 'cos I is black?' like Ali G. That didn't win him many friends with the staff.
"He was trying to be funny. He had a very cheeky attitude to the teachers. If the teacher he said it to sent him to the head he'd just say it again to the head - he didn't care.
"I reckon he was trying to show off to get attention from some of the girls. Ali G was really popular at the time so people did think it was pretty funny the first few times, but it did get old after a while."
The friend, who asked not to be named, also said Amin "had a screw loose" on the field. He said: "He had a habit of putting in really bad challenges - sliding tackles and 'clothesline' tackles and stuff.
"He was a nice enough guy but he definitely had a screw loose. Even at school or on nights out he was always getting into fights - usually over nothing, like someone knocking a drink."
A few short status updates on what is believed to be Amin's Facebook page hint at his turn towards Islamic militancy.
In November 2012 he wrote: "Truly loving the Prophet Muhammed ... could only be demonstrated by following His teachings, not by singing His praises." The post received 10 "likes" and one comment.
On April 20, a post declaring "Muhammad is the messages of Allah and those who are with him are SEVERE AGAINST THE DISBELIEVES AND MERCIFUL AMONG THEMSELVES (48.29)" attracted four 'likes.
And on Wednesday he added a photograph of a mysterious tapestry hanging in a darkened room.
Another former schoolmate said: "He was pretty cocky and arrogant when I played there with him. As a player I didn't rate him much. He was speedy but that was about it. He hadn't played rugby before and I think he thought it was like American Football - he was always going in on high tackles that could have done real damage to someone.
"After the games he could be a bit of an outcast. We used to like to go for a few drinks if we won but he obviously didn't join us.
"Ramadan also muddled up his rugby. We were all on diets to bulk up but if you're not eating all day then you can't really do that."
He added: "When I saw the papers I thought he was just another idiot like Abu Hamza, and it took a while to click that I actually knew this guy.
"It makes me really angry to think that he's doing this after everything the country gave to him - without us he wouldn't even have an education."