Amanda Young is accused of giving 14 times the amount of an anti-psychotic drug prescribed to Joshua Gafney at his home in Yeovil in February 2012.
Bristol Crown Court heard a second character witness say Ms Young was "dedicated to her profession".
The 40-year-old, of Yeovil, denies manslaughter by gross negligence.
Her defence also told jurors at least two other nurses had given Mr Gafney similar doses in the days leading up his death.
But the prosecution said Ms Young was responsible for the fatal dose.
In his closing statement, Mr John Price, prosecuting, said: "Is this rough justice? Is this fair because all these nurses got confused by the labelling on the bottles?"
He then answered his own question, saying: "No, this wasn't rough justice because the overdoses given by the other nurses hadn't caused Joshua Gafney's death."
He added Ms Young was "grossly negligent" and this was manslaughter.
Elizabeth Marsh QC, defending, said Ms Young's actions were "a mistake by a compassionate and focussed nurse" but that mistake had caused his death.
She went on to say "there was not a day that had gone by since, that Amanda Young had not dwelled on the mistake she had made".
She added: "Maybe it's not as simple as the prosecution are suggesting. Maybe administering this medication wasn't simple."
The jury also heard it was "unusual" for that particular drug to be administered in a liquid form and none of the nurses was familiar with it.
The judge is expected to sum up on Tuesday and send the jury out to consider their verdict.
A mental health nurse who gave a lethal overdose of a prescription drug to a patient was "kind and empathetic", a court was told.