"What happened to Shane to turn him into such an unrecognisable person that night? "Was it because he didn't drink or do drug that his system couldn't cope with the antidepressants?
"Or can depression melt your brain if it gets that bad? Will we ever know?
"All I know is that Shane wouldn't hurt a fly but turned unrecognisable that night for some reason."
Shane attacked Sebastian, 22 with a knife he had bought in a 24-hour shop, at the Creane family home in Bray, Co Wicklow.
His ex-girlfriend Jennifer Hannigan who had started going out with Sebastian, suffered a knife wound to the back.
Sebastian's older brother Dillon suffered eight stab wounds but he also escaped with his life.
Grief-stricken Leonie said she wanted listeners to know "who my son was and what he was like before the 16th of August".
She added: "Shane had it all. A stable home, a lovely flat in Dalkey, his own car, college going well and family and friends who loved him unconditionally.
"He gave half his grant money to a homeless man he befriended, which meant we had to pay his rent but that was normal."
She said Shane began to show symptoms of depression in the months before the attack.
Leonie explained: "He was to go to Calcutta for the summer but foolishly, maybe, told his mentor there how down he was feeling about the break-up with Jen.
"The decision was made to defer his trip until next year. He was at home with me the day he got the phone call telling him the trip to Calcutta was deferred.
"I told him to go on the internet and book himself a round-the-world trip as he had ended up now with a free summer."
Shane returned within two weeks saying he missed his family and friends.
Leonie continued: "I drove him to the doctor and made him get some anti-depressants which he didn't want to take, but I insisted."
She now fears the drugs contributed to the sudden burst of violence that brought such tragedy to three families.
The medication is expected to be brought up during the inquest into Shane's death.
Speaking about Shane, RTE radio host Gerry Ryan said: "He was a full human being, admittedly tortured... in need of help maybe that he didn't get."