Cho's first thoughts about school shooting

I saw in a place When cho heard about columbine, he had a urge to repeat columbine but when he exactly turned homicdial and what was the process?

I don't think you(or everyone else) can get clear and proper answers to these questions. The only peson , who knew all of them for sure,is dead. That's what makes events like Columbine,Virginia Tech and Sandy Hook even worse - people who did it, had killed themselves and deprived others of any chance to establish a full picture of what was really going on with them,that made them do so.
So,as you understand, all of us can only wonder
Personally, I think that Cho had made a long way to become a killer, and , certainly , this process took a decent amout of time and included many stages. I suspect, that he had reached the point of no return somewhere between late 2005 and early 2006,after he failed to get proper mental health care, and,possibly, felt that nobody can help him.I think that he felt great anger and hopelessness, and comitting a murder-suicide seemed to him to be the only option.
Again,it's all my thoughts and opinions on what had possibly happened to him. I have nothing but to speculate, when it comes to such things

Based on what I read Cho's homicidal and suicidal thought probably started around middle school when he was like 13-15.

This is what the vtech report says on cho mental health history
------------------------------------------------------------------------
Then, in March 1999,
when Cho was in the spring semester of eighth
grade, his art therapist observed a change in
his behavior. He began depicting tunnels and
caves in his art. In and of themselves, those
symbols were not cause for alarm, but Cho
also suddenly became more withdrawn and
showed symptoms of depression. In that context,
the therapist felt that the tunnels and
caves were red flags. She was concerned and
asked him whether he had any suicidal or
homicidal thoughts
----------------------------------------------------------------------
The following month, April 1999, the murders
at Columbine High School occurred. Shortly
thereafter, Cho wrote a disturbing paper in
English class that drew quick reaction from
his teacher. Cho’s written words expressed
generalized thoughts of suicide and homicide,
indicating that “he wanted to repeat Columbine,”
according to someone familiar with the
situation.
--------------------------------------------------------------------------------

So Cho probably had suicidal and homicidal thoughts since he was an adolescent.

I think what made him homicidal was his mental illness he was described as having severe social anxiety and selective mutism. I think he wanted to fit in with people but could not and got frustrated and ended up doing what he did.

I think when he lived with his parent his parent sort of took him to therapy and gave him anti phychotic medication but when he left for virginia tech his family was no longer there to see how he was doing mentally, and what his action were.

For example his parent took him to a shrink when he wrote an essay on repeating columbine, but as he got older his behavior escalated he started stalking people, got arrested by the police and nobody was informed since he was an adult.

I think Cho had the thought for a long time but he did not really want to prepare or plan to do it until eary 2007 like February when he bought his guns.

Probably shortly after Columbine.