Marcellus did not die a violent death and was formerly venerated at Rome as a Confessor in the original meaning of the word.  Because of his mildness in reconciling sinners by the Sacrament of Penance the austere heretical party raised a tumult.  The tyrant Maxentius then condemned Marcellus to exile from the trials of which he subsequently died.  He many thus be rightly called a martyr because of his sufferings.
(Source: Fr. Lasance, The New Roman Missal)