(a) As long as the speaker smiles hypocritically, his anger with his foe continues to grow. Thus, his smiles are acting upon his anger like sunshine, helping it to grow.
(b) ‘Deceitful’ means ‘deliberately done in order to fool someone’. The speaker pretends to be friendly with his enemy by behaving in a very sweet manner.
(c) The speaker was afraid to express his anger with his enemy. Hence he pretended to be friendly and happy with him. This pretence only made his anger grow.