Rod Reiss was aware of what would happen if he became a titan, and ate Eren. Even if you are anime-only it explains that Rod was a coward, and was scared of inheriting the titan ability since the moment he looked into his brothers eyes and "realized" what it meant. He knew that once he became a titan, he would die 13 years later, and that his mind would be altered to the beliefs by the power of the 1st king. He was so afraid of these things that he was willing to sacrifice his own children to prevent himself from succumbing to these curses. The reason that he sacrificed his own children, and not random people he could've easily persuaded was because he was aware that in order to access the "true power" of the founding titan within Eren, you must be of the royal bloodline. This is why he was so intent on Historia. After she denies him they reveal that he had little to no love for Historia herself, but more so the blood in her that could've prevented him -- or at least given him an excuse temporarily for not inheriting the founding titan.