By Brian Ellsworth MIAMI (Reuters) -Prosecutors portrayed the man who killed 17 people at a Florida high school in 2018 as a calculating and brutal murderer, while his defense attorney pleaded for mercy before jurors begin deliberating on whether he should be sentenced to death. Nikolas Cruz, 24, pleaded guilty last year to premeditated murder at Marjory Stoneman Douglas High School in Parkland, about 30 miles (50 km) north of Fort Lauderdale. The Valentine’s Day school shooting was among the deadliest in U.S. history. The 12-member jury will begin sequestered deliberations on Wednesday. Browa…