Three preliminary questions. First, can you read French? That's about the most important part. Second, it would help to know what you intend this for, too, as that can alter the reading I might be able to suggest. And finally, can you be more precise as to your chronology? 'Before the start of the Terror' can range from anywhere prior to 1793, really; do you mean his stances on these as a deputy, or before the revolution itself, etc.?

To begin with, have a read of a general biography -- McPhee, Scurr, Thompson, etc. -- and glean some ideas from that. Since you're interested in his positions, especially in regard to potential regicide, see Jordan's King's Trial and Revolutionary Career of Maximilien Robespierre. The best compilation of his views during the Revolution itself -- though I stress that these changed with the times, including his perspectives on violence, the death penalty, virtue, terror, friendship, women, and so on -- is the 10-volume Oeuvres de Maximilien Robespierre as republished in 2000 by the Société des études robespierristes. For your interests, this would mean volumes III to IX, which feature his correspondences with his brother, journaux, and speeches until July 1793 (see http://ahrf.revues.org/2502?file=1 for more specific detailed of the contents). You might also be interested in his pre-revolutionary views as expounded in his legal cases and presentations for various Academy contests, for which you should look at the first two volumes (Robespierre à Arras) of the Oeuvres, or the first few chapters of McPhee's biography for a detailed summary.

Should these be unavailable to you, a lot is on the internet, although you should be careful as to the particular claimed source. Vermorel & Cournol's 1867 collection is accessible online at http://fr.wikisource.org/wiki/Œuvres_de_Robespierre, and see especially Sur le parti à prendre à l’égard de Louis XVI for his views on the King's trial. There are also plenty of his discours on Gutenberg, e.g. http://www.gutenberg.org/ebooks/29775, and the Archives parlementaires are all available online, anyway, either via Gallica or the new Stanford FR Digital Archives.